The problem is for 40k they obviously used 100 dollar bills for the seals and for a diff cover gasket. 100 dollar bills don't seal too good.
if the diff cover is flat try locktite 518 gasket eliminator.
that is better than any yamabond or other gasket "maker"
Trust me i rebuilt a briggs and stratton 5 hp years ago and it almost started when i was done!