Tom, I would also suggest looking for some of the older Honda Fourtrax models (the ones that were air cooled). They were based on the engines from Honda's XR line of motorcycles which are just about impossible to break. I'm not as familiar with the Yamaha stuff, or nuances of quads in general (my background is motocross), but the quads often borrow from that mfr's motorcycle line so there is some crossover, and many pitfalls are the same. I believe both Honda and Yamaha had quads that borrowed from trail bikes (mostly air cooled four strokes in a more reliable state of tune) and some that borrowed from the motocross side (water cooled, more power, less reliable especially if maintained by someone thinking oil changes are optional).
As MX (race) bikes go, Yamaha has a good reputation for durability, but that's like saying that a pro stock drag race engine is reliable compared to a top fuel car. If neglected on oil and air filter changes as well as top end maintainance, modern four stroke motocross bikes can grenade in rather spectacular fashion. When they do, depending on what goes, even if you do the work it's ridiculous what stuff costs (not by airplane standards, but as a percentage of what the quad/bike is worth). Most share oil between the gearbox and engine, so when one goes it causes issues with the other.
In general, if you see an air cooled four stroke engine it is likely based on that mfr's trail bikes and in a milder state of tune. Feel free to shoot me a message if you think I can help. I raced motocross for 20+ years, the last 10 were on four strokes and rule #1 by my dad was I worked on my own bike.