Longevity on a 49cc Huasheng engine for me has been over 20,000 miles and several punishing years with a lot of over-revving just to see what they can take. Haven't thrown a rod yet and I own a few of these little engines.
The oldest HS 49cc I own has the original Grubee 4G tranny with the tapered shaft and is only on the second clutch, it has all the high performance mods available on it and well over 20K miles on it. I have only replaced the belt once.....it didn't even need replacing.
I can vouch for the reliability of these little 4-stroke power plants, just maintain them with frequent oil changes and ride them HARD. Adjust the valves if you feel the need and don't look back!