That old tractor used a Honda G400 engine, an old side-valve design, 406cc in displacement.
Do you need the whole engine, or do you have the original motor? Some parts are still available, but entire engines are long gone. It's possible a more modern OHV motor may work/fit. You might try and find a Honda Engine dealer who can help you look; here's a link to find one in your area: Honda Engine Dealer Locater
Finally, here's another discussion forum with LOTS of old Honda tractor folks; a lot of knowledge here, and maybe some pointers to a gently used engine...
