What big bore kit will work on 1987 fourtrax 250

 

The 1980s TRX 250 was a nice smooth bike with a rather weird front suspension, I guess Honda were working out what suited an off road 4 wheeler back then and tried the "dune buggy" suspension idea.. However it was never going to be a racer and I would guess, if any tuning parts were available, whoever made them would have given up by the mid 1990s. If you want a fast bike, buy a sports bike. Use your bike for what it is, or sell it on.

If you are referring to the sports bikes 250X or 250R then racing parts were available, but again doubtful if they still are, for bikes over 30 years old.