I had an electric trials bike for a while, it was very impressive in most ways.
Lack of power/ torque won't be a problem, there is a MX bike due to start being delivered anytime now that has various power settings but capable of producing 80hp. With electric motors that power can come in right from cracking the throttle if you want it to. The Trials bike and I believe the new MX bike came with assorted power maps as well so it can be equivalent to anything from 80cc to 500cc.
I've seen electric bikes in flat track races as well and there is no doubt the lack of noise seems to detract from the spectacle. However with lack of noise comes a lack of complaints from neighbours as well so I am sure it opens up the possibility of motorcycle sports happening within towns again which is something that Speedway desperately misses. Indoor Speedway on a variety of surfaces is also far more possible.
I would imagine one problem will be cost for a while yet, the Trials bikes are all but double the price of a petrol one and the Stark Varg MX bike is £10k which is a lot nearer a sensible price but still expensive for a Speedway bike. There is a possibility though that you could have series from kids through to GP's using exactly the same setup with different maps and bikes lasting for years with next to no maintenance.
A lot of plusses but the big minus could be whether fans would ever accept quiet Speedway. Following the fuss about silencers and then further noise reduction (some seem to think the bikes are quiet now) I very much doubt that they will.