First thing to do is determine what type of airplane you are going to build or buy. Then check what manufacturer
has the size you want. I personally always recommend one with a ball bearing supported crank and that covers almost all the manufacturers. As louw covered almost all the bases on the makers all you have to do is fix it in your budget. I have OS, ball & bushing types, as well as Magnum, K&B, Norvel and a few others I haven't had out of the box yet. Check with the pilots at your local field and you can find as many different answers as there are people. At least you can see which ones are hard to start, don't idle well and don't have top end RPM. There is also a variable to that as well. Some guys just don't know how to run their engines.