So many engines to from which to choose. Best advice above was to check with other pilots at your field. My first engine was a Baby Cyclone in 1936. I bought it used. Can't tell you how many engines I've had since, both good and some poor. The thing I've noticed laely seems to be that too many pilots say "Run one tank of fuel through it and it will break in while you're flying". Maybe I'm just old fashioned but I still believe that the clue to engine life and performance is a good break in. Manufacturers always include an owners manual that will tell you all you need to know about break in. Read it. If you're talking diesels, I've never been able to get one to run. They are used extensively in Europe and by a few in the US and all the people who have mastered them are very pleased with the little beasts. Someday I'll take a course in diessels and learn what I don't know about them.