There are a few ways to get oil in a cylinder, gasket failure or cracked castings between an oil gallery and a cylinder, the two studs either side of the cam chain tunnel on the intake side, are under pressure and can blow oil out. Return galleries can sometimes get broken into as well, cylinder pressure can blow into the gallery, and pull oil in on the intake stroke. (exhaust coming out the crankcase breathers, oil blowing out everywhere else it can.)
The valve guide seals on the intake valves letting oil pull into the port, the port when running, has a vacuum trying to pull oil through the guide, can also just flow under gravity or capillary action when not running. Exhaust guide seals can leak as well, but usually just by gravity/capillary when stopped. Usually seen as a puff of smoke when starting and then reducing, unless completely failed with badly worn guides.
Or the cylinder, because of rings, either gummed up, worn beyond tolerance or broken, or damage to the cylinder wall or piston.
I'm going to bet on gummed up oil rings, either stuck out and worn off, or glued in and not touching the sides, plus the hard guide seals. (hoping anyway.)
On a separate note, I'm sure you know this, and took due care.
Whilst happy to see hammers and chisels in action on all things I can, I would say, the mating surfaces of finely machined parts is probably one of the rare times I would advise against them.
I doubt that there's any damage done, but little nicks in the sides of studs from being hit, create a failure point, burrs hold things apart, gouges create gaps between things that gasket goo cannot always fill.
