1982 Kawasaki 1100 Spectre issues....

Runs great BUT does need work.

1) Bike needs a new base gasket installed.

2) Last fall, suddenly while riding down the highway, the transmission just automatically down-shifted from 5th...down to 4th gear. I was really not sure what was going on with it. Still unclear, but have been told by several people about it possibly having a bent shift fork. I need to know how exactly, step by step how to remove the shift forks???

Also, what is the best service manual I can get for this bike?? Please advise. Any info helps. Thanks a million!!!