Mulled this idea over myself a few times over the years as electric cars can outperform pretty much anything on the road powered by fossil fuels -
https://www.youtube.com/watch?v=apoeGMWF17c
Apart from mind-bending acceleration, electric motors are also far simpler than conventional engines, meaning they're theoretically more reliable.
About the only downsides would be the same as for current production electric vehicles: range-anxiety, finding a suitable place to recharge mid journey and the length of time it takes to recharge the exhausted batteries.
But, for an amateur builder the cost and complexity of the components has to be a factor and it's that that's put me off wanting to actually build one.
A decent motor, motor controller and suitable batteries are likely to set you back around £8-10k, and while you can pick up a suitable petrol powered donor for a couple of hundred quid (or less), that makes much more financial sense.
As for the range anxiety, I'd toyed with the idea of a hybrid charging system using a small petrol powered generator to kick in at around half charge to top up the batteries on the move, but as I'm no electrical engineer I've really no idea if that's feasible.
On the legal side, you can plop an electric drivetrain into an existing car, change the details on the V5 just as if you'd changed to a different conventional engine, and benefit from road tax exemption.
The rules for an electric kit car would also be exactly the same as for any other kit car, so IVA if it uses a bespoke or modified chassis and radically altered (rebody) rules if it uses an unmodified classic chassis.
Hope that hasn't put you off building one as I'd love to see somebody put one together.
BTW, what have you been building for the last 15 years? Given your moniker, could it be a Dino by any chance?