After seeing the pic and the thin wall thickness of the tube I would simply simply run a hacksaw through the crack, v it out tig weld it up, file it flat and bend a piece of flat ally around it and weld that to it to strengthen it up a bit / adding some extra wall thickness.
Looking at it I'm not at all surprised its cracked, it's very thin and the loadings in that area will be high from torsional stresses. It won't take long for fatigue to creep in over time.
I'm trained in tig welding and an engineer by trade, so that's how I'd do it - but I wouldn't have to pay someone to do it as I have all the tools so it wouldnt cost me anything to fix. if uou have a mate with a tig he'll fix it no problem for a minimal fee.
You might find a cheap swing arm else where but if you plan on keeping the bike then a repair will be stronger than new swingarm in the long run. In fact I'd probably do both sides cause the other is bound to go soon too.
Without seeing a picture - my initial thought would be to 'v' out the crack, weld it up with mutiple passes, then ream it out say 2mm larger and fit a permantly locktited steel threaded insert machined to the bearing o.d.
Not saying you should do the same but I just used Gas Gas uk for my engine rebuild parts. Google their website. They will have 95% or more of the parts on the shelf ready to ship, they are very fast with their deliveries and you can have an account with them too which is nice (just quote your unique reference number and you can empty your wallet super duper quickly).
You will likely pay more for the parts (genuine) and good service then other places but if you're lazy like me it's extremely convenient.
You can source bearings from bearing manufacturers, pistons from piston 3rd party manufactures if you shop around and you can cut your own gaskets etc to save money, but it'll really hinder your time frame if you want to get it done quickly.
It won't be cheap rebuilding your motor, I recall mine costing circa
It's a uk company. This stuff is what we use for all our diesel fuel lines at the engineering company I work for. It's also approved by the MoD. It has loads if uses though not just on pipes, very versatile chemical.
If needs be have your wheel trued and ensure all spoke nuts are tight.
Use a brass wire brush to clean the wheel.
Degrease the wheel
Get a tin of Heldite jointing compound.
Put a blob of it on each spoke nut.
It will last years and years and years and years and years... It will never leak. Period.
If you need to change a spoke no problem. You remove the tire as usual and scrape the Heldite off. Refit new spoke, tighten it to the correct torque setting (Tighten it till it 'pings' the same sound as all the others).
Heldite also acts as a thread locker so they won't come loose.
If you're wheel needs to be trued in future well it's not a job that any old Tom duck or Harry can do anyway. Just remove your tire, scrape off the Heldite and give it to your wheel truer. When you get it back re-seal with Heldite and continue trialling head ache and leak free for many more years to come.
Tire bands are crap because:
A). Rubber gets hard and porous over time which = leaks.
when ally and steel are in contact together (i.e where a spoke connects) moisture present will cause electrolysis / galvanic action (corrosion) - it may not necessarily corrode near the spoke it could be anywhere on the wheel.
Not seen a diagram for the 2013 model but I no doubt suspect you will have ha cable coming up from the flywheel case via a ac/dc rectifier which goes to a 12 volt regulator. This will go on to supply the cooling fan and you can tap into this if there isn't all ready a spare terminal (that would normally be used for lights and instrument panel.
It's just that I started reading what was an interesting story of the restoration / re birth of a bike and how great it looks etc, when I finished reading I couldn't wait to see the beast in all it's glory, but in the end all i got to see was a tip of a mudguard and a bit of your "ahem" helmet .
Sorry I have a really dry sense of humor is all ???????
As an engineer I wouldn't reccomend using a mig to weld anything on a trial bike unless it was aluminum and you are a good welder.
If its steel or 4130 CoMo and you care about your bike it should be tigged or oxy/accetelene welded. 4130 also needs a specific filler rod suitable for super steels.
Dont get me wrong You will get away with a mig welder and regular steel welding wire but it won't be as strong as the parent metal or look abywhere near as neat as it could so you might be welding it again in the near future when it starts to crack. It'll be okay for a quick repair, just not the best.
You change the piston if the piston is worn / damaged or if your barrel is worn which can be determined / measured with a bore gauge in wich case you would need an oversize piston.
when ally and steel are in contact together (i.e where a spoke connects) moisture present will cause electrolysis / galvanic action (corrosion) - it may not necessarily corrode near the spoke it could be anywhere on the wheel. 
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07 270 Rev 3 With Unusual Crack?
in Beta
Posted · Edited by tankygsy
After seeing the pic and the thin wall thickness of the tube I would simply simply run a hacksaw through the crack, v it out tig weld it up, file it flat and bend a piece of flat ally around it and weld that to it to strengthen it up a bit / adding some extra wall thickness.
Looking at it I'm not at all surprised its cracked, it's very thin and the loadings in that area will be high from torsional stresses. It won't take long for fatigue to creep in over time.
I'm trained in tig welding and an engineer by trade, so that's how I'd do it - but I wouldn't have to pay someone to do it as I have all the tools so it wouldnt cost me anything to fix. if uou have a mate with a tig he'll fix it no problem for a minimal fee.
You might find a cheap swing arm else where but if you plan on keeping the bike then a repair will be stronger than new swingarm in the long run. In fact I'd probably do both sides cause the other is bound to go soon too.