Another idea would be a K&B Sportster 20 (or maybe 28) which can be found on ebay quite cheap if you are patient. The 20 can swing very large props without over heating. I know this because after reading about Stu Richmond running a 20 on a 12x6 (not a typo!) I bought one and carefully broke it in on all-castor oil fuel and worked my way up from a 9x4 at first to a 12x4. It ran smoothly and quietly on the 12x4 and throttled fine too, making 7350 rpm at full throttle. I didn't try going to a higher pitch. It was good on smaller props too. I got 11,000 on an MA 10x4 G3, and on a Zinger 11x4 I got 9150.
I have run diesels on some of my planes but tired of the smell and mess. I still have some small diesels which I don't mind. I think the Sportster 20 is a good alternative to a diesel in that size. I image the 28 would be fine too, but I don't know.
Jim