Worst part usually is removing the transmission and the repairs are normally fairly easy! I used Three Bond 1104 or 1194(comparable to Yamabond) at the shop for engines and transmissions.This is close to what the factories use. You can get these at most auto stores. THREE BOND 1104NEO GASKET CEMENT SEALER CAR,TRUCK,MOTORBIKE,ATV ENGINE,GEARBOX on eBay!
Another good sealant is Hylomar,but fairly expensive for a small tube.