Be fortunate enough to get a good engine to begin with, put a quality upper needle bearing in it, dont try to blow it up, tune the carb properly, run a quality oil in it at the recommended mix and chances are you'll get purdy darn good service out of it.
All of these engines are a bit of a crap shoot, but there are some that seem to be better put together than others.
So far Ive had 4 out of 6 engines hold up teally well, so I dont complain.
Ive got my best performance and smoothestrunning engines from Dax so far, but Ive heard there are others that have provided these same qualities.
I run Opti2 @ 100:1 in all of mine with excellent results, Im not an everyday rider, but when I ride I do long distances much of the time, 40 miles round trip isn't uncommon, I have heads on my engines that do allow them to run cooler and I honestly think if you run them hard for long distances this is a big plus.