If you cut the frame right behind the front wheel mounts and lengthen the frame out 3 feet a small block Chevy v8 will fit nice.��
Unless it is a John Deere 1963 or 1964 model it will be a lost cause. Those are worth a couple thousand but after that the value drops quit a bit. I had a 1967 model with snowblower, mower, blade weights and it all ran that I paid $500 for. 20 years later I repainted the tractor and all the attachments and was able t sell everything for maybe $600. And it was all in good shape. But, your experience may be different that mine.