To be perfectly honest, it depends on how much work you're willing to pay me to do, and what you want the engine to do. For me to "go all-out" and completely rebuild the entire engine (all new bearings, gaskets and seals) and replace any worn, broken, or questionable parts, and me performing all of my tweaking, it can get expensive. I prefer to build the engine as new as is practical, but if parts don't need to be replaced then I don't replace them.
Just some examples would be chipped gears, worn clutches, questionable bearings, questionable crank (at rod bearing), etc.
Keeping in mind that it's my reputation in the engines that I build, I don't take chances on anything inside of them.
The Airsal kitted Tomos engines I've built have been clocked (by radar or police) at 55+mph.
Butnut's Maxi E50 engine I built with an Airsal, and was radared at 50mph, fighting a 45mph cross-wind, and running really rich.
I hope this answers your question.