Good ol used Briggs 2 hp Flat head engine. This is from when Briggs were made to last.
Still fires right up 2nd pull and runs like champ. This thing is a sweet heart and it's pretty clean with all the original decals. Just serviced it's got a new air filter and spark plug.
Tank has a tiny bit of surface rust inside but really not bad, keep it full of fuel and you should be fine or could easily be coated. This motor has a 5/8" tapped shaft that is 2 1/4" long. Came off an old walk behind edger that just sat in a barn for years.