The easiest way to do it is to buy a front or rear "electric hub kit", sold by many online e-bike shops, and they are quite cheap on ebay. The front kit is easier to fit. You can buy them with or without li-ion or lifepo4 batteries. I'm using two different brands of kit, both rated at 48v 1000w. Running on 14.4ah old li-ion laptop batteries, no pedaling on flat road, one kit peaks at about 27A, 1000w, and peaks at about 40kmh, but the other peaks at about 46A, 1.7kw and peaks at about 50kmh. I think the quicker one is made by "sun e-bike", but neither have clear branding on them. I bought them both on ebay. I've only used lipo's once, but the high discharge rate gave the full 27A on 8ah of batteries. I've only been playing around with these kits for a couple of weeks now, and I'll do some more experimenting with my rc lipo's. The 250w, 350w and 500w kits are cheaper. If you're travelling at 40kmh without pedaling the whole time, you'll probably need a pretty big battery, I'm guessing at least 20-25ah, to get 40km range. I'd say a 350w kit plus you pedaling will probably give you 30-35kmh, and 40km range on a 10-15ah battery. There are a few Youtube videos of people testing out their systems, and will give you a pretty good idea of what to expect.