If the rear seal is mounted on a removable plate you can remove the plate, drive the seal out, drive the new one in, put the plate back on. If not, you'll have to use the screw method above but be VERY careful not to damage the metal on the crank or block. I have the best luck tapping a nail into the old seal in the middle between the crank and block. Resist the temptation to use a drill for this, as a nail with not do near the damage as a drill bit if you do get into crucial metal by mistake. Once you drive a nail through the seal, cross thread a wood screw into the hole, and claw hammer the seal out. Tapping the new seal in can be a bitch, I have the best luck using a ring compressor, adjust it to the seal's diameter, put it against the seal and wood block the other side and carefully hammer it in. Remember to put a little oil on the seal so it won't burn up, crank side. Note, a seal puller usually cannot be used in this application because the crank is in the way.