That's a better basis to start with indeed :thumb:
I would use a mid drive motor with chain to the rear wheel. That way you can use different gear ratio's to find the best one for acceleration vs speed. And you won't need a new rear wheel/brake. That would look much better than different wheels front vs rear. With the mid drive electric motor in place of the gasoline engine it will be nicely in the air flow for cooling (=more power input without overheating).
The Eastgem d4500 kit would fit very well I think:
http://www.eastgem.net/d4500-k-1.html
With a continuous power rating of 4,5kW (6hp) it would be double the output of the original two stroke engine. And it will probably have a peak power capability of double that for a few seconds to accelerate seriously quick 8)
Then you only need to try and find battery cells that can provide the power that the controller and motor can handle. You can start with a few or cheap cells and program the controller to the max. that the pack can provide. And later when you can afford better or more cells you can upgrade the controller settings for more amps.