I'm not taking any responsibility if you break it
, but if it was me and I was sure it was push-on pull-off (I dont know with any certainty) then I'd probably spray the crap out of it with PB Blaster every day for a week, tap it lightly with a hammer and a piece of wood after spraying to vibrate it a little, then after a week I'd use that same wood to drive it in more. Like 2 taps or so, then try to pull it out with a wrench wedged in there, prying it gently or tapping the wrench with a hammer to tap it out.
Any way you can pull the engine to make it easier?
Also, if worst comes to worst you can poke the stuff back into the engine and just change the oil. It's probably had worse than that in it in the last 50 years. They're tough engines.