The reliability factor of these little engines generally follows how high of sustained RPM's your running them. The factory usually designs the motors to stay around 8,000rpm. When you modify them (air cleaner, carb, reeds, pipe) you are basically allowing them to breath more freely. Raising the rpm's gets you that extra 5-10 MPH @ say, 10,000 rpm. There are not a lot of gains to be had in the low-end of the power band which is why everyone is always asking about a good low-end pipe around here. It's easier to open them up.
While an engine can be designed & built for sustained higher rpm's, most of these little guys aren't and only have a 10mm wrist pin bearing, where a 12mm would be the preferred size.
The other way to get higher speeds is to gear them differently (taller ratio). Even at that, you have to have the HP to pull the higher gear range or have a long distance to get there and the low-end fun factor suffers. You have to find the compromise that fits your riding style and excitement expectations.
Use a good quality oil, keep them jetted properly, the ignition timed correctly and that's the best you can hope for. Some brands may also be inherently better than others.