I've ran into two tractors that seemed to be contaminated with gasoline. Both surprisingly ran fine except one missed at high rpms. I don't know what the contamination levels were, i.e. how much % was gas vs. diesel fuel. It's hard to tell anymore (at least for me) a clear difference between diesel and gasoline by scent alone. I can definitely tell a difference by feel. Gasoline will feel much cooler on the skin and dry quickly while diesel will feel much more slippery and hang around longer.
Since you're looking at a replacement, the engines that will fit your machine are the Mitsubishi KE70 (power downgrade) or KE75 engines. Mitsubishi K3A and K3B (3 cyl) engines could maybe be modified to work (hydraulic pump is on opposite side and I believe the blocks are longer) but none of those engines can be easily had from my experience. It may be best to watch for a parts tractor with a decent motor for the same amount you mentioned (not easy, not fast, but possible), at least you will get some extra parts also. If you need it in a hurry you might have to pay the price.