Your opening a can of worms trying to put something different in, ..........dont get me wrong it has been done many times, especially with majors as they have a seperate sub frame, but by engineers/ mechanics with knowledge and access to all the tools.
I have a friend who bought a second hand engine, the same one but out of a compressor, turned out the sump was different and wouldnt fit round the front axle, went and got a 2nd hand sump , but then the oil pickup would fit, even with the help of a mechanic who helped me do a full not and bolt rebuild of a fergy, they eventually gave up and sold the whole lot on.
Id guess your cheapest/easiest options are engine rebuild (i think kits are available for the major) or 2nd hand engine from another tractor------but my thoughts are always this with 2nd hand engines.... people dont tend to remove engines unless they are broke ! To many unknowns to make the call, whats wrong with your current engine, what kind of order is the tractor in are all factors.
Just my thoughts, others may be able to advise differently.