Well, I got 2500 miles (2480 or something) on my HT 65cc, which was the best. I got it from DAX.
My first motor was something I got on EBAY, I don't know the source, I got 1500 miles on it before the wrist pin bearings went.
I then went and got a DAX hearing good things about them, obviously there are other good companies. The first DAX wen 400 miles and the wrist pin bearings went - DAX sent me another one and let me keep the first for "parts" (geez, I got a lot of "parts"...).
The second DAX was the one I got 2500 miles on. It was still running but noisy and losing power. I was able to wiggle the crankshaft a little, so I am pretty sure the main bearings are going.
Most problems I have had otherwise due to my errors - not getting the good motor mounts, not rubber mounting the motor (I use pieces of inner tubes), the sock I use as a prefilter for the air filter getting caught in the drive sprocket (YIKES - now I tie the end of it to the frame). A fair number of flat tires, innertubes I get off of junk bicycles are often better than WalMart - at least they have held air at SOME point.
I can go on and on, and usually do too much I think
Chris