We receintly had one of our more senior club members bring in his engine collection. He had some single cylinder as well as 5 and 6 cylinder radial 4 stroke engines from the 40s and 50s. They were hand made by a guy in up-state New York. Some of them even had true spark plugs and distributor cap/point setups. Not very reliable from what I understand, but very impressive for their time. For my money, I go with Saito. You get a reliable, quality engine without paying for the OS name. Not that OS isn't a good engine, but for 10-20% less you can get the same size and quality Saito. All of my 2 strokes are OS. As to why they are so expensive - a lot of small moving parts with tight tolerances. Take a 2 stroke 90 size apart and set it next to a 4 stroke 90 size. 5 times the parts and work go into making a 4 stroke.