when u changed the sprockets did u match them up with original?

if you have changed the gearing the engine may not be able to match up and will seem under powered (i changed my gearing on my gsxr so i had more acceleration but 20mph ish less top end)

did you check the spark plug gap?? does the engine run ok?

does it splutter or hesitate to rev? any odd smoke out of exhaust??

i belive the engine u have is from a honda xlr (could be wrong)