[What's_nitro?]

What is the rated HP on the Baja engine? For a ballpark figure take that a multiply it by 750. Since 1 HP = 750 W, that will give you the sustained wattage (~5 seconds) you would want from an electric setup to achieve the same performance. So if it's 3 HP, then 3 * 750 = 2250- meaning the system should be capable of sustained bursts to 2250 W.

This is just a guide. Personally, I'd want a system capable of 4000 W in a vehicle as heavy as the Baja. But see what the stock engine is rated for and that will give you an idea. What voltage do you plan on running? 10s LiPo sounds like a good starting point. I'm sure someone here has experience with converting the Baja so they can chime in as well.