Heard of issues with the yx and zonga gearboxes quite a few years back (sloppy tollerances) but nothing bad in the last few years. The yx140 is a decent motor these days. That being said, lifan 140s have always been rock solid, but the yx140 feels a little faster (can only go from seat of pants feel)
Though i rode a GPX125 back to back with a yx140 that made the 140 feel SO much slower. I don't know if anyone is importing/selling the gpx125 in the US, punches way above its weight and LOVES to rev, if i had $500aus to spare at the moment, I'd buy one of those