If it is a 731, it should have an air-cooled Deutz diesel engine in it. A 730 would be Wisconsin powered.
Parts are available for Wisconsin engines, but depending on what engine it is and what is wrong with it, it could be an expensive repair.
Without a running engine, it would be difficult to determine it the machine has more issues that just engine problems.
What are you plans? Fix it and keep it, or fix it and sell it?
Unless you can get it to run a bit before buying it, I think that you would be better off passing on this one, especially if the owner won't even counter-offer.
My $0.02 - SR