You may be able to use these adapters...I would advise that you be ready to drill a hole in each one that will hopefully be able to match an existing hole in the frame dropouts though for stability so they do not shift on you.
This works with motorized or standard bicycles and allows you to install a disk brake if you don't have the necessary mounts. No more rim or coaster brakes.
www.bikeberry.com
On my bike, (steel frame ONLY), i had to "stretch" the rear forks apart by hand, about 1/2 an inch to put the rear wheel with sprocket/rotor attached, and then use a spare 15MM axle nut on the sprocket/rotor inside of the drop down, threaded to the axle with blue locktite, to maintain that amount of "stretch" to accommodate the amount of of clearance space the rotor needs so as not to be pressed against the frame.
It helps to have a buddy with a strong pair of hands to help in this proceedure like I did...lol.
Do NOT attempt to stretch if you have an aluminum frame as it will crack on you.
You may also be able to do something like I adapted later on so I can R&R the rear wheel by myself though, this method is perfect if your using mag wheels but might be a little tricky with spoked rims.
Here is the tool I adapted:
It’s nice to have an unusually strong friend around to help you stretch out the rear frame for modified rear wheel accommodation.
motoredbikes.com
And this is how the tool works:
It’s nice to have an unusually strong friend around to help you stretch out the rear frame for modified rear wheel accommodation.
motoredbikes.com