I made a bunch of progress this week:
I pulled the engine because it's easier to replace the starter that way. While it was off I took the mounting plate and trimmed it to better fit the Bolens, about 1.5" off each side. Now the sides of the plate line up with the frame rails. The new starter is larger than the old one, but it fits just fine. I assume it's a Chinese copy since it was only $35 on ebay. I've purchased from the seller before, a motor for my plow setup, good stuff.
Everything went back together smoothly and I installed the new carb. Since the engine hasn't been run in years I put some oil in the cylinders and turned it over with the plugs out. I installed the plugs, tied the old tank to the steering wheel and started it up. Once the oil burned off the smoking stopped and it ran smooth. But then .... I decided to punch it to see how it performed and oil started leaking onto the exhaust. I assume the head gasket blew, oh well, that's easy enough to fix.
I need to get a new throttle cable since the original is too short. The choke cable was dead when I got the tractor so that needs to be replaced as well. I've decided to keep the battery in front and put a gas tank in the rear. The battery space is right near the exhaust and I'd rather not have a tank of gas there.
The picture below shows the test rig, ugly, but effective: