It could be the crank seals leaking, as far as I know we haven't found a source for the original style ones, but they can be replaced with O-rings of the appropriate size (there are two different sizes of the crankshafts & bearings depending on the age of the engine).
The first two engines I worked on ran OK with leaky exhaust collector gaskets (I had no replacements at the time), but I now replace them as they usually leak after removing the cylinder.
I think I have at least one earlier engine that refuses to run properly despite being completely stripped and rebuilt.
David