Maybe not nitrous, but you can find hi perf parts for a standard Briggs & Stratton motor.
A side note; I was at a M/C dealer show years ago and saw a chopper powered by a B & S motor. It was about a 20hp(originally), that had been worked on to boost it to about 35. Mikuni CV carb, Supertrapp exhaust, lots of chrome...
It was billed as "The Other Milwaukee Legend".