Question on a kohler

My friend is looking for a project for him and his boys and he is considering a 1968 Cub Cadet 124. This is not a Cub Cadet question though. The guy replaced the original engine with a 15hp kohler (after doing some mods to the tractor). I do not know the engine model or age. My question is, are there any single cylinder, 15hp kohlers to stay away from? If we could cover any and all years so I am prepared when I find out what it has, that would be great. Thanks.