In the affordable 1/2A + aisle, you've got the AP .061 Wasp. Its an RC engine, but you can easily lock the throttle open with a piece of paperclip wire. I got 2 of them and have been very satisfied. You need to use low pitch props to get them to rev like they want, but I've flown planes with 36" wingspan on one engine and 44" span on 2. (Note the 44" span plane would fly on one engine, but would not take off on one.)
Think I gave something like $40 each for the engines. I use a Sullivan 1 oz slant tank with them and run muffler pressure.