The MVVS engines might be a good choice. the .61/.77/.91 are all around 550 gr (~20 oz.) and you can use them as rear or side exhaust since they have a detachable cylinder (you have to decide before break-in). All three displacements have the same dimensions. I think they use the same case. Also being European make, they were designed to run on 0-5% nitro = cheap fuel.

http://www.justengines.unseen.org/mvvs.htm
http://www.justengines.unseen.org/mvvsdims.htm