Hi, I am a newbie to the e-bike world. I am converting an old bmx style bike with my son as a project. I am mechanically and electrically inclined but advice is appreciated.
I am making it a trike with the original drive wheel on the back along with one on the other side. I removed the pedal crank as it won't be a hybrid. I was going to buy a motor/controller/throttle kit but the only thing I am not sure of is the sprocket. I want to use the bike sprocket on the rear with the existing chain. I have the front sprocket off the pedal crank. Is there a way to reuse that sprocket with the chain or is there a specific motor sprocket that will accept a bike chain? Or do I have to replace the sprocket on the drive wheel with one that accepts a #25 (I believe) chain? I will need to fabricate brackets and such to hold it in the proper place so that is not an issue (unless there are pre-fabriacted ones that can be modified)
Also what size motor and battery would be enough to drive the bike at a speed comparable to a scooter if not faster? I was thinking 48 V 500 A. If any of this is way out there let me know, thanks....

You are saying you are building an electric motorcycle? If so, you need to find a different forum since this is about bicycles but I don't know about that.
I have used a 500 watt (I am assuming that is what you meant, not amps) inline motor and it has enough power to get up our hills but only if I pedal. If you don't have the ability to pedal (a big mistake in my opinion, since it adds so much to the versatility), I doubt a 500 watt motor would get you far at any speed. You will probably have to get it registered and licensed without the ability to pedal. Even my 750 watt motor which is plenty fast and powerful on my recumbent, wouldn't be good enough without pedaling.

ok, thanks I will reconsider the pedaling option but this is just meant as something for my son to drive down the street, not really a serious project (more along the line of a sit down electric scooter). I was mainly concerned about a motor I could use with the existing sprocket and bike chain.

I would highly recommend that you check your state and local regulations before you get to far into your build. Most states have simular regulations as the fed regs. In most cases you can not exceed 20MPH on flat ground when powered by motor alone and you can not legally exceed 750 rated watts. And an ebike is legally defind as having 2 or 3 wheels and must have working peddles.

Bob