When running 4 strokes I have built my own engines and i have used one of the top builders in the country, and I can tell you it was way cheaper to pay that top national builder than trying to find myself and messing so many parts and engines. Besides there are tools needed to have a good engine properly built that are way so expensive that you can't afford buying unless you make a living out of it, other than the knowledge and experience of some of those builders is something you will never reach. In fact I think their work is worth much more than what we pay here in the US, and if you don't believe me check try to find out what top 2 stroke engine builders used to charge in Europe back in the 80s and 90s.