Engines and parts for the OH180 are very hard to find, as you're finding out. I don't think they produced nearly as many of them as they did the 160, and the builders of pulling garden tractors snapped up what they could years ago as well. I'm no expert on four-strokes, but I remember some of the builders using the head from a 180 on the 160 to make it breath better.
Stuck in the seventies- not in the swamp.
(6) Attex, a Hustler, a Super Swamp Fox, (2) Tricarts, (3) Tri-sports, a Sno-co trike, 3 Dunecycles, and a Starcraft! ...so far