2 things Stu:
First, although I believe regular motorcycles 'could' be lighter (as the new KTM free ride shows), the electric bikes I've dealt with are all almost HALF the weight of the their ICE counterparts. Electric motors and batteries are very heavy but are more compact. Where an ICE trials bike has every available mm wedged together, an electric trials bike has free space like when you put a 50cc engine in a full sized motocrosser! The trend I've seen in the E-bikes uses mostly high-end down-hill bicycle components. E-bikes are smaller overall and lighter. The advantages are obvious.
The second thing regards racing. I don't foresee the end of ICE, however, I think you'll EVENTUALLY see more importance in racing rules geared toward conservation. As I have suggested elsewhere on this site, having a simple fuel allotment (like in NASCAR, F1 and others) is only the beginning. I (if I were king) would open the rules up and put in restrictions based on national goals such as emissions - this season we'll track your total CO emissions via your issued ECU (as in F1) and you can't go over X for the season. You'll see racing managers slowing racers on the track (as you see in F1) managing resources.
I like box rules in racing with simple goals to obtain. If you want wings, ground effects, turbo supercharged nitro burning wankle engines, that is fine - but your simple goals are: 6 engines per season, 3 gearboxes, emissions of X, X gallons of X type fuel or X gallons of Y type fuel and 100 pairs of tires (20 of which are rain). Good luck and see you at the starting line.