Unless it's an iron head, it does have seats. But the seats aren't the problem, they can show bad compression, but they won't blow oil. The valve guides are where the excess crankcase pressure will come from, or the rings.
Not having read all the history on the engine. How much time has it been since it as overhauled? How many miles after overhaul?
Blow by is actually the gases blowing out of the combustion chamber into the crankcase causing high pressure in the crankcase. It can blow any oil in there out the breather. The only places it can blow by is bad or broken rings, worn valve guides, or maybe a crack or hole in a piston.
If the crankcase is flooded with oil (wet sumping) because of oil leaking into the crankcase from the oil tank or or seeping through the oil pump, then normal crankcase pressure will blow that oil out the breather. that's why you check the crankcase for excess oil before you worry about the top end of the engine. You should be able to drain the excess out of the crankcase. There should be a drain plug under the crankshaft of the engine, usually where the sump pump (the return oil pump, it may have a screen there) is. Take that one out. There should be a little oil in there, but not much. Maybe a t-spoon or so. If you get several ounces out it's probably wet sumped. Dump that oil back in the oil tank and try starting it again. The blowing oil should clear up within a minute or so after starting. If it doesn't, check the level of oil in the crankcase and see if it's filled again. The return oil pump may be the culprit.
Bare