First, the keyway is damaged on the crankshaft:
[Linked Image]


It isn't really good enough to use at this point. Was the engine on a Scott Bonnar 45 mower? That mower has a strong tendency to cause this kind of damage, but by the time it has happened, the engine support rails are usually cracked or even broken, as well.

IIRC that oil seal can only be removed by drilling a couple of holes in its metal cover and using them to prise it out. If you are going to proceed with an engine overhaul, that process is a standard part of it. However without knowing more about the mower, and your overall enthusiasm for persisting with this engine, I can't advise you whether it is worth proceeding.