Re: Splitting the crankcase
I'm not saying anything bad about Roger, he does know his stuff, but that's some more advanced stuff that you can do once you learn the basics of these engines a little more... or be ready to have a lot of nice long conversations every step of the way so you don't mess anything up.
For your current issues, the pistons have small locating pins in the ring grooves so the ring can't be put on upside down and so the ring gaps don't migrate toward the intake and exhaust ports where it would surely destroy a ring or worse. The rings should go into the cylinder with very little force, if it stops going in it's most likely that the ring slipped over that little pin in the ring groove.
Greg is right about the seals and you should replace them if they look to be damaged, you can pry them out with a screwdriver and the new ones should push in fairly easily with your thumbs, or just the right sized socket and a gentle tap... again, use as little force as possible.
Another method of getting your bearings and seals in and out is by using a socket and a long bolt and nut where you can draw the seals or bearings in or out by tightening the nut, no hammering necessary with this method, you'll need one socket that's bigger than the part to be drawn into place or pulled out, and one that's slightly smaller, as well as a bolt long enough to go thru everything and a nut... and a few washers to keep the bolt/nuts from going into the sockets. This is a far more gentle method for getting the new bearings or seals into place since there's no need to hammer. Plastic mallets are better than hammers for assembly too if you can't press them in or out with the sockets and a long bolt method.
Hopefully some of this will be of help to you.