RE: Electric conversion of SPAD GNAT
Ahhh, very good!
Here is a link to a similar size plane at Hobby Lobby called the [link=http://www.hobby-lobby.com/eagle.htm]Potensky Eagle[/link]. Instead of being made from Coroplast, it is made from EPP foam.
This size plane can fly on a wide range of motors depending upon your price range and performance expectations. On the low end, a geared Speed 300 motor would work. The most common is the GWS EPS-300C or 350C power systems at your local hobby shop or any on-line GWS distributor.
For a higher power, higher cost, no maintenance option, a non-geared outrunner like the [link=http://www.hobby-lobby.com/brushless_external.htm]PJS 550E[/link] would really give you unlimited power. It comes with the motor mount that could easily be attached to a Coroplast firewall. I've done this on a slightly smaller model.
Good luck!