I took one look at this bike and thought it looked a lot like the first motorbike I built, which was on an Electra Coaster 7d Frame.
If you look at my Electra Coaster 7 Project
page on Tabblo you can see how I solved the long frame issue. Basically I extended the rear mounts with threaded rod but found I needed to add a vertical brace to dampen vibrations. It's all on my page.
You should be able to do something similar with the bike you have.