I don't think a leaky rear crankcase cover gasket would cause an engine to run rich, would it? Wouldn't that make it run lean?

I'd take a look at the ring in the engine that won't run right. Sometimes when a motor hasn't been run in a while the ring can stick and cause problems.

Also, if the engine stopped the last time it ran with a valve spring compressed, and it has sat for that long time without being rotated, the spring may have lost a lot of it's tension, and that could cause it to miss and run rough. Just a thought.

David