You definitely want something with a governor on it. Most diesels should have one. Automotive engines will fall on their nose, unless you find an old 6 cyl. truck engine with a gov. Or, put a belt type driven governor on it.
An adapter plate will have to be made to fit between the engine block, and your tractors clutch bell housing. Depending on what the engine is off of, the crankshaft may need to be bored, and a proper bushing inserted for the input shaft, to keep it in place. Then some sort of cross member across the front of the engine to carry it. Then comes to adapting to the original radiator, or replacing it, to be able to cool the engine properly. In the long run, if you have to have all of those pieces made, it'd be less expensive to rebuild the engine in it, with a lot less hassle.