Early motors had a flawed counter shaft and crankcase half design that led to transmission failure very early on in most bikes. The fix kit and later motors corrected that flaw, and are about as reliable as any other Lifan 120 / 125 ilk motor (same bottom end, just a shorter stroke setup.). Occasionally you will see an early motor that holds up just fine with the flawed setup, but it's pretty rare. Any repair that does not include an updated crankcase half and counter shaft is going to fail in short order in the same way, so verify what was done if a seller mentions they fixed the tranny.
There are a few ways of dealing with shifting problems if you do experience the dreaded false neutral issue. SDG has a shift star update kit that I'm using in my Pro Mini with good luck so far, or Dr ATV offers an all up shift drum conversion kit.
The motor is a bit tough to build at the moment, because of the 3v head. Cams are available, but bore kits are hard to find. You can drop a Lifan big valve head on however as the stud spacing is the same, which will allow you to take advantage of 54m and 56mm pistons with ease.
Supposedly the later frames are made from better grade metal, and gussetted better, but I don't know when those changes went into effect.