We have a Yerf-dog 90cc 2-stroke. We've had it for almost 3 years now and have had pretty good luck with it.
My 10 year old son and 12 year old daughter both ride it, along with their cousins, and it has withstood most of what they have dished out.
From what I can tell, it is a generic, far eastern knock off. I can't even tell you where it was made. That said, the engine is pretty bulletproof and my mechanic agrees. Our local small engine repair shop is a "certified" Yerf-dog servicer, which means that they have access to parts (they do a lot of go- cart work). We've never had any problem that they couldn't fix for cheap.
I have tried to get parts from Yerf-dog directly, without much luck. I was literally told that the parts were "on a slow boat from China." We couldn't wait, so found other solutions.
When we first bought the quad, I ordered a service manual from Yerf-dog. It has not been much help. Perhaps some information was lost in translation, also the photos and diagrams were not too well done.
While it does not have the fit and finish of our other quads (Honda,Yamaha and Suzuki), it starts reliably and has never let us down. We replaced the battery and a brake lever and repaired an oil tank bracket, but that is it.
I know that there are a lot of low-end import quads out there. Because of their presence in the go-cart market, they more have more staying power than some and have at least some dealers/service shops, etc. Also, Yerf-dog is selling through Sam's Club/Walmart and has been for a couple of years. So, they must be providing some level of service in order to keep that relationship active.
I don't know if any of this helps, but at least it provides some Yerf-dog feedback.