Don,
Here in Hoosier Land, we tend to use gassers for our WW1 birds. 1/4 scale will run from 23cc to 40cc. A lot of guys do their own conversions, the Homelite 25cc makes an excellent engine. Some of us have converted 4 strokers, a friend flies a 1/4 scale Fokker D-VII with a converted 30cc Briggs and Stratton 4 stroke and I have converted a 35cc Honda 4 stroke, that will probably find it's way into my friends 1/4 scale Nieuport 17.
The Zenoah G-23, G-26 and G-38 are all very popular
My list:
1/4 BUSA Fokker D-VIII Evolution 26 cc
1/4 scale Eindecker E-1 Zenoah 26 cc
1/4 BUSA SE5A ZDZ 40 cc
1/4 Nieuport 17 Brison 2.440 cc gasser
steve
www.hoosierdawnpatrol.com