The bore and stroke is all mapped out for one engine and one engine only. If you get lucky and find a piston with the right dimensions you still need to worry about clearing the counterbalance on the crank and making sure any holes are properly aligned. Not to mention some pistons have oil grooves and some don't. Then too you also have to worry about which con-rod to use, and what size the bore of the holes are on that con-rod.
It's like trying to take a 350 Chevy engine and bolting up a Ford 350 intake...... The parts weren't made to fit and therefor won't without major, expensive, innefficient, and difficult modifications. Just search the major sites for some parts, your LHS is always the best bet (Support the places you race!), but next to that I would say try one of the major internet retailers... from there turn to Ebay.