I haven't replaced a seal in one, but if you can see the seal complete, a hook type seal puller or even to zip a sheet screw into it and tug it out may work. Sometimes if you get the hole started, you can grind the end of the sheet screw flat and let the screw push it out.

Tip; take fine emery cloth strip and clean the whole exposed crankshaft before pulling the old seal. Let any remaining oil out of the crankcase. They have seal picks at the auto parts store. After getting the old seal down off the crank, use an oily clean cloth to wipe the crank and remove any dirt/grit. Put the new seal over the crank and get it into place. You have to use a piece of pipe long enough to clear the end of the crank and of the right diameter to press the seal evenly into place. They say you can use PVC but it must have a square cut on the business end.