Williams Bros makes a few large scale aircooled aircraft engines. The company was sold awhile ago, and I believe the only ones re-released are the Le Rhone WW1 rotary and the Pratt and Whitney Wasp Junior. While Wiliams bros kits vary in quality, their engines are among their best kits.
There are a couple of aftermarket suppliers that make auto engines in 1:25 scale. I think Ross Gibson is one of the brand names. Best place to ask about those is the Autos forum.
There was a kit, long out of production, of an Allison turboprop engine by a major mfg, I think maybe Revell but not sure.
Yeah Ross Gibson makes killer resin engines, and a ton of different ones. You pay for it big time though. When an engine is as much or more than the kit… I tend to have an issue forking out any money for that.