There are plenty of automotive engines that have been converted for use as aircraft engines. Subaru EA81 flat-4s, for one. I've also seen VW Beetle engines an

There are plenty of automotive engines that have been converted for use as aircraft engines. Subaru EA81 flat-4s, for one. I've also seen VW Beetle engines and Mazda rotaries converted for experimental aircraft use. Snowmobiles and jetskis aren't automobiles, but many ultralight engines are based on snowmobile/jetski engine designs. (Rotax is very common, though I've seen at least one BD-5 with a Polaris snowmobile engine converted for aircraft use.)