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.

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.

All parts available from Gas Gas motors uk (do web search).

520 chain - correct.

Re: flywheel weight, most bolt on. My S3 flywheel weight on my 280 gasser is bolted in with 2 Allen key bolts, I'm fairly sure it's the sane on a 125.

If its running sweet leave it alone.
If you really want to find out what the condition is I suggest you purchase / beg /steal / burrow a bore gauge and a compression tester.
Your method of compression testing by bump starting down a hill is meaningless.
I would suspect there is absolutely nothing wrong with your rings after 200 hrs. I'd be more concerned about the gearbox from all that bump starting!

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 £250-350 (off the very approximate top of my head in this manner (ill doubke check later to give you s mire accurate cost) but I did do a very thorough rebuild with all new bearings, 1 or 2 gears in the box and a couple if gear forks (mine were bent and worn so couldbt straighten) and these can be quite expensive. Given the age and overall condition of your motor I wouldn't be surprised if you'd be replacing similar parts.


Tel for gas gas uk is 01298 766813.