Boy oh boy, does that restored plunger frame bring back memories... of sliding down the road on the front half of the chassis after the driveshaft and transmission grenaded.
I should have done some more work to it before I rode it. It had sat from 1961 until 1992, rotting away under a tarp in some old coots chicken shed.
The pic of the 1941 R12 did inspire me to go pull the ancient boxer twin engine out of the creek today. No idea what it is, has a combination generator/plunger operated electric starter, Tillotson 1bbl down draft carburetor, car type flywheel, transmission bolts on like an old top loader (have not found the trans yet)... and it has strange manifolds, BMW are 1 into 1, this one has 2 into 1 intake and exhaust. Only identifier is an L inside a shield on the cast iron fuel pump drive housing...
Toes, come get it and make yourself a 1/2 scale R series...