Neat idea but I think there would be better or easier motors to retrofit to that tractor. The VW motor was meant basically to mount strictly buy its bell housing bolts, 4 of them. some had pulley end bolt bosses as well if they came from a bus. It's also a wide motor compared to what belonged in there, so it's going to look odd at the very least. Also keep in mind that the VW engine has it's oil drain and screen at the very bottom center of the engine, making oil changes a challenge in this case. It also doesn't survive well without proper air flow, so the cooling shrouds must stay, even if modified somehow.
If your luck enough to find a bus or type II block, with the pulley side bolt bosses, you will have to make up a set of mounts on both ends, aligning the crank center with the tractors driveshaft. after that, it's a matter of linkage.
I would probably opt for going water cooled though, even diesel before going to a motor that more than likely going to require lots of maintenance to keep running. Also keep in mind that most VW engine topped out in the 50 to 53 hp range, had no OEM oil filters, and were single carbureted, with a rather odd generator or alternator arrangement. Adding additional items to be driven by the motor can also be a challenge. Then you have to deal with the exhaust, keeping in mind that all that tin on that motor was designed to prevent exhaust heat from rising up and being recirculated into the cooling air system. This won't work in an open engine compartment situation. A custom set of exhaust pipes and some care to direct their heat away from the motor and the driver will be in order as well.
I'd probably look more towards a small inline 3 or 4 diesel as a motor option if I were to completely repower something.
The sky is the limit with those motors, but their not a cheap to build as they used to be. Check out Geneberg.com for any high performance parts, they were always tops in quality when building an engine to last.
It's been 20 years since I was really into those things. I used to have a barn full of VW parts and motors. It began with a few stock bugs, then a few baja bugs, and then all out sand rail buggies and drag racing.
I sold off most of it about 20 years ago, but still have a lot of engine parts left packed away, and probably a few engines here and there.
We usually stuck to the 1971-72 dual port bus engines, the added front mount and larger oil pump boss were the main reasons, and the aluminum was better in those years. I think it's been a good 10 or more years since I've even seen a VW on the road now around here. They seem to have vanished.