The basic engine is pretty much the same, but you will have to shell out some pretty big $$$ for the proper block adapter, flywheel, and starter. You will need to fab some mounts for each side in front.
Also, tractor engines usually don't have the Bosch VE pump (load governor vs. speed governor) so driving it would be weird....also you may not be able to very easily turn up the power.
If you end up buying the right pump, you'll also need the proper line, too. This stuff all adds up.
Besides....every time I have tried to deal with tractor salvage places, I have found their prices to be WAAAAAY high compared to similar items somewhere else. You can usually find similar items in other places for a lot less. I'll bet they want $4k for a 4BT from a Case or Kubota tractor.