The stock TLR intake, ignition and exhaust systems kill the power pretty much stone dead. Stock unmodified bikes have around 8bhp at the rear wheel, and are not that great if you intend to ride in competitive events.
In terms of cost modifying the stock exhaust is FOC for anyone who can use a MIG welder, and can get a used car exhaust to provide the parts needed. A very low cost item is the Chinese CDI unit, which will fit a TLR after simple wiring changes, and while working best with higher compression motors, do improve the stock low compression ones as well.
The following are the best possible modifications for a stock TLR, where there is any intention to use it in serious competition:
1) Exhaust system alterations as outlined earlier.
2) Removal of flame trap and intake silencer elements from stock air box, fitment of foam pod filter onto stub fabricated to fit into intake hose.
3) Replacement of very often badly worn OE carb, with new Taiwanese direct OE replacement.
4) Fit Chinese pitbike CDI unit, which provides stronger spark, more spark advance at lower RPM, and does not have the retard function built into the OE units, which on the dyno acts exactly like a rev limiter, restricting rear wheel power to something like 8bhp.
None of the above changes are required on bikes not being used competitively, and I get the feeling the restrictions on the TLR were designed by Honda to help make the machines bulletproof reliable, and difficult/impossible to damage, even if being used for long periods at high rpm, during trail or road riding.
All in all the TLR is a bike which can be easily and dramatically improved for very little money, and as this is the case I find it surprising that most people seem happy to ride the stockers, which arent really that much fun, when compared back to back with a modified machine?