True! But for many popular airframe/engine combinations there are companies which hold supplemental type certificates (STCs) for the conversion, see for instanc

True! But for many popular airframe/engine combinations there are companies which hold supplemental type certificates (STCs) for the conversion, see for instance Petersen Aviation who have a dedicated website for STCs covering conversion to auto fuel: https://www.autofuelstc.com/ https://www.autofuelstc.com/