Basically, the G5 is the new UCE engine in an electra frame - has a kickstart. The C5 model is a new design - including tinwork and frame - with the UCE - no kickstart.
To make it a little more confusing, RE produced a G5 Miitary model for the US market in 2009. Replaced in 2010 with the C5 Military... new model C5's coming out next year or so... think they'll all have a kicker...
I haven't looked REAL close, but it looks like a lot of the frame and chassis parts are NOT interchangable between the C5/G5... where a lot of the G5 stuff could be shared with the "classic" design... I'll let the experts weigh in...
Mike and S&S in Michigan