i have a 25hp mariner with a blown powerhead. would it be practical to drop a air cooled vertical shaft engine on it like the ones they have on a pro drive? has any body ever tried this? i know there is a big difference in rpms from an outboard to an air cooled engine. could the difference in rpm be made up by changing the propeller? i dont know how far i will get with this project but any knowledge or experience you guys might have with this would be really helpful

It's been done , there are videos floating around on it. You should try your search function above ^^^.