If it chokes on full throttle you need a new exhaust. This one is most likely blocked internally. (they partly vibrate to bits inside) The seal on the clutch lever shaft is easy to change. Check oil level first as there might be too much oil in it. (Let engine run for a minute or so, kill engine, keep bike upright and check dipstick. ) If the original gear lever is still on there take it off and replace it with a cheap as you can find steel one from a Yamaha XT500 or such. You need to bend it a little but at least the shaft won't break or bend when you drop the bike on the original lever.... ask me how I know) The rear brake possibly has air in it if you can't see any leaks. Put new brake fluid in on both front and rear brakes.