On the 100's, some early units were missing or needed a O-ring under the carb from what I have gathered. I have several 46NT's and I have run the heck out of them, only needing bearing changes, they're still as strong as they ever were. I don't know if they have recently corrected the bearing problems that many of the engine have in common with OS, TT and others. and that is the use of cheap bearings.(OS does use high quality bearings in Heli and ducted fan engines) Inspect at the end of the day by pulling and tuning the prop. If it's a little rough/gritty get on order some stainless bearings that use plastic retainers from RC Bearings.com. (fair prices)
I have not had much luck with ceramic hybrid bearings.