OK I have a TW200 with a bent valve. I know a valves cheap and everything but I also have a TTR225 engine and TTR230 engine in my shop. The 230 engines a runner but I can visibly see the countershaft is too short. I have a 225 engine that had a motor mount bolt seized in it, when hit with an impact to loosen it I broke the front motor mount clean off the engine. This engine could be robbed for parts. I've read that the 225 top end can go on but the cases need bored, that's more work than changing a valve. However it didn't mention if I could just stick this 225 head straight on to the TW200 cylinder?
I honestly can't be sure. I would say you'll get an answer soon but I would go all or nothing on it. Either replace the valve or bore the cases and go bigger. I have a feeling you can make the swap with the stuff you've got on hand, but standby for better info on that one.