My B has a CR engine in it. It appears to be one of two things.
1. A CE engine from a combine - mine had the combine oil deflector trough in it.
2. A factory replacement block / engine.
All the normal CE parts weem to bolt right on, so I'm assuming it's the later 125cid motor. I didn't have to go into the long block but I suspect the CA parts (only sleeve/piston combo available) will fit.
Of course, my "1939" has foot brakes, belt/PTO/hydraulics, electric start, etc.