I know how to replace the seal. You have to remove the transmission from the engine, and take it apart. On the inside of the transmission casing there are three 10mm bolts that hold the input shaft seal in place. Remove it and take it somewhere to find a new seal.
I know this because I did it just a few days ago. Now I need to find out where to get a couple new gears from. The local tranny shop says he can't get new gears anymore so he is looking for a core with decent gears.
I don't want to hijack the thread, but does anybody know where I can buy new gears for the 4 sp? What is the model number for it? It IS a Muncie, am I correct?