I'd agree with KC on the bearings, probably no reason to mess with them. You will want to check all the bearings and make sure nothing's going out though – check for roughness or looseness (you'll feel just a little play, but don't worry about if they're consistent). Splitting the case isn't too big a deal, and you definitely want to replace the seals. It's not worth splitting the case when the bottom end and seals aren't in question, but on an old sled that's never been rebuilt, that's not the case. Regarding pistons, OEM are best if you can get them. Thing is, unless you intend to put a lot of miles on them, they're probably not worth the extra cost. Aftermarket pistons will last the rest of the sled's life in most cases.
02 Polaris 600 RMK
Project sleds: 89 Indy Trail, 94 AC Pantera