Right... listen carefully....
Use the 300 frame and swinging arm that you have. Before you put anything together strip the swinging arm bearings are grease/replace. You will need to buy some rear shocks. I recommend BETOR or better. Use the complete front end from the 301 including the headstock bearings. You may need to make up a spacer as the 301 headstock is slightly longer than the 300 and may not tighten up. Use your 301 rear wheel but you will need to get a 300 rear brake plate to make it fit. I think spindle is same. Use your 301 engine & carb. Should be a straight fit. Not sure if the sump plate will fit or not but probably can be made to fit. You will need for a 300 - an exhaust, airbox, tank, seat, rear mud guard, brake pedal & rod. See Richard Allen for parts. Helpful but expensive.
exhaust £100
airbox £100
tank £50
seat £100
rear mud guard £30
brake pedal & rod £40
You should then have an arguably reasonably twinshock that is capably of winning trials. There are MANY tweaks that you can still do but that should get you started.