YOU WILL HAVE SLIGHTLY DIFFERNT COMP AND YOU WILL HAVE TO REJET THAT CYL AND ITS NOT THAT YOU CAN'T DO IT. IWOULD, IN MY OPION, PATCH HER UP AND UP GRADE TO A NEWER TWIN. SKI-DOO HAD ALOT OF TROUBLE WITH THEIR EARLY TRIPLES AND WHATS TO SAY YOU DUMP ALOT OF MONEY IN IT AND SOMETHING ELSE CRAPS. THEY HAD PROBLEMS WITH THEIR CRANKS AND CRANK BEARINGS
not sure how bad it would be... I know of someone who got a did that to a polaris triple. They replaced one piston, had it bored out, not sure how much, and then had the other 2 honed (don't know how to spell it) and there was a bit of a compression difference but only about 5psi. Better off asking whoever is gonna do the work for you... they should know.