Don't forget!
The P85 Fuji is the only direct application for the 503F, for the 600 the taper has to be re bored to the Rotax crank, this is why the Comet copy made new for the Ski-Doo is the least expensive option.
The taper bore is so important to be accurate for balance, have heard of some engines vibrating off their engine mounting bolts, carnage!