I fix my YS myself, I have had my 110 completely into atoms and replaced the oil seal and main bearing (it runs like new again in case anyone wonders). I always thought this was a very complicated engine, but everyone comfortably with normal 4 strokes will have no probles fixing an YS themselves. Yes there are a few more parts, but it`s really not that difficult. But one has to know some basics about how a YS operates, like W8ye says, even the valve cover gasket is critical.
There`s no way I`m sending off my engine halfway around the globe (and wait for it for ages) and pay 165$ for a little maintenance