It seems like everyone (and their dogs) is calling their motorized bikes "Jessie James", so I found one of thousands to discuss...
There are many options for this kind of sprocket mount, but here are the ones I know of and have tried with opinions...
1) Bolted to the spokes.
This is by far the easiest option, but also the worst. All I can say is... don't bother.
2) Bolted to a disc brake hub.
This works well, and can take perhaps 2 or more horsepower. I did this by hand filing out a sprocket from a 1 piece crank...
3) Custom machined hub.
Best option for sure. With all the gas bike kits around, you can surely get one for under $50 from the many online sellers.
A proper sprocket hub with 36 spoke holes could easily take 4 or more horsepower on a decent 20 inch BMX rim.
I dont know if I will take the pedal out.. I think yes.
The method with the disc brake adapter seems nice but I wonder if there is a bearing attach to the sprocket? When the engine goes ahead its fine but if the engine does not turn and the wheel yes, without bearing.. what gonna happen?
You say it will be better on a decent 20in bmx rim but does the large tire will fit?
Sorry for my english I usually speak french and this is my first diy bike so maybe I ask stupid question but my sprocket make me crazy.. lol
I wish I could help, but I think that your questions would be properly answered on the forum for the supplier of the gas engine bike.
You will see that this corner of the web isn't much into "gassers", mainly because I am all about electric!
I cannot really advise based on what you have shown, as I do not know these components.
The freewheel part will be the difficult part for sure.
The main problem being that the pawls creating the ratcheting drive are not really designed to handle the extreme torque of a motor like that. 1000 watts continuous may be able to deliver 3 times that during acceleration, so that is many more horsepower than would normally be seen under human power.
I have seen mid drives attempt to use standard BMX freewheels in a jackshaft arrangement, but these simply wont last on a motor of your size.
So your goal is to be able to use human only power at times?
That is certainly the difficult part.
Look up "sprague clutch". I used one on a motor / human project once.
These are not exactly budget items, but the can handle the torque.
The only other options I can think of...
1) Just let the motor freewheel. If the controller is "open" when idle, the friction won't be too bad. Good enough at least for "legal" reasons that might force you to have human power.
2) Multiple BMX freewheels on a jackshaft. This one is tricky though, because if you don't align all pawls to click at once, 100 freewheels will be no better than just one - they will just break one after the other. Kind of like when a Karate master in a demo smashes 20 board in a row with small gaps between them. Not impressive at all, because it's no different than smashing one board!