In the picture below, look at what the guy has in his hand...That is the type of spacer that comes with my type of mag wheels when they come with the sprocket and disk brake set ups...That spacer is what is usually used because of room constraints...Be aware it will only give you just enough space to put the disk caliper in place when you are using the 36 tooth sprocket or smaller because anything bigger won't allow the caliper enough space without hitting the caliper.
If you use a sprocket any bigger that 36 tooth, you would have to heat up the frame and/or weld a different configuraion to stretch out the rear drop down support tubings to make it all wide enough to fit.
On my Hyper, I was blessed enough to be able to hand stretch those out the half inch I needed to fit the wheel with sprocket, spacer, and disk.
If you have a steel frame, you can do any of what I just said, It its an aluminum frame, your SOL.
"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/spacer/rotor attached, and then use a spare 15MM axle nut in between the sprocket/rotor and the inside of the left 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."