The seal is pretty easy to change. Unscrew the pump knob. Below that you'll find a rubber washer that acts as a bumper. Remove that as well. At this point you may be able to dig the old seal out with an ice pick or other pointy object. If that works, grease the piston rod (silicone grease) and slide the new seal down and press it into place. Slide the rubber washer onto the shaft, screw the knob back on and you're done.
If you can't dig the old seal out with the piston rod in place you'll need to remove the cylinder from the base and slide the piston and rod out. If you do that you might as well put a full kit in.
Go to Groco.net, click on Service Manuals
and with a little looking you'll find a nice exploded drawing of the toilet and also detailed, slightly confusing, rebuild
If you get stuck, take Peggie's advice and call Groco. They have a reputation for good customer service