The best thing I can tell you with the TTR230 is cc's, exhaust and cam.
There is now a 250 cc kit available at
Engine's Only
It should be mated with some other mods for maximum gain:
1)air box opened up and aftermarket filter
2)performance exhaust
3)performance camshaft
4)consider lightening flywheel
Stock hp is about 16-17 uncorked and about 24-25 hp after the mods listed above. There are also significant torque gains.
You should really look at this situation though IMHO, as a TTR230 with 24-25 hp and almost twice the torque will overcome the stock chassis and suspension and will need to be significantly addressed.
If you want a fun "play" bike or "capable trail" bike...go ahead and do the mods.
If you want a "performance oriented trail bike" or "off road racer" look elsewhere.