I've heard the Thunder Tiger F-91S described as the perfect engine for the Hog Bipe. It's available from Infinite Hobbies for $184.99 and shipping is free for orders over $150:
http://www.infinitehobbies.com/moreinfo.php?pid=5276
The Magnum RFS .91 would probably also be an excellent fit for the Hog Bipe, and HobbyPeople.net still has them on sale for $149.99:
http://www.hobbypeople.net/gallery/210970.asp
They're still on backorder at the moment, but I imagine they'll get backfills handled soon enough.