(snip)
On many engines, you need only replace this seal and go merrily on your
way. But occasionally (and in my case) it's caused by the crankshaft
moving forward and back as you press the clutch. And THAT is caused by the
crank's thrust bearing seat being badly worn-- usually so much so that you
can no longer adjust it out by shimming the flywheel as per the normal
procedure. And THAT means that not only do you have to rebuild the entire
engine, but you also have to shell out extra for a new crankcase, since
the one you have is shot and can't be salvaged.

(end snip)

Actually, you can re-use that worn case. What needs to be done is have it 
"thrust-cut", which will true up the bearing surface. Then, new main bearings 
that have a thicker shoulder are installed to take up the difference. Just like 
you can buy main bearings with oversize diameters for an align-bore, you can get 
ones with the thick shoulders.

Sure, a new case would be better.Buuuut, if you don't have the cash....
(Local guy here gets $40.00 for the align bore, and another $30 for a thrust 
cut)
TomC