ORIGINAL: A1
Unless you have a mechanical degree or have been a mechanic for the past 10 years, DO NOT buy a YS engine as your second engine. It's the hot rod of engines that takes a lot of understanding to operate. But it takes no skill at all to mess up. I own two and hated both of them, until I figured out the process of how they work. It's been the school of Hard Knocks. The most flying they did was on a UPS jet going back to service for the first couple of years. Read YS Support Forum to see for yourself.
I have to disagree here. I too heard that they were difficult and required some kind of voodoo to operate. Once I bought a couple though I realized that they were no big deal. Yes, a properly plumbed fuel system that is sealed well is critical. That isn't rocket science though, just follow the instructions that come with the engine. The needle valve is no different than any other 4-stroke glow engine. Adjust it for maximum RPM then back off a few clicks and it is good to go. The low speed needle is reversed on some (out is lean, in is rich) but that's no big deal either; it tells you this in the instructions. The main thing is to not mess with the regulator screw; that's where I see a lot of people screw up. I even run them on 15% fuel with no problems at all.
The power these engines produce is unbelievable and when set up and tuned properly they are extremely reliable.
That being said, any Saito, OS, Magnum or other decent engine in the correct size range will fly the plane great.