First I must say you did a very clean fit, I like how well it's resting in the frame, and something I was thinking about. More pictures please!
You are doing a single speed set up, OK so for a brake, how feasible is it to put a brake rotor on a geared shaft so that when the brake is engaged the rear wheel is slowed down by the gear along the chain.
It's like a jackahafted brake.
Can always attempt a disk brake on the right side if you are choosing to remove the chain on that side, considering the pedal width against the engine...
They have hub clamp adapters, it clamps to the hub with arms comming out and they one way or another hold the brake rotor. Then you adapt a disk brake caliper and its mount onto the frame to do the dirty work.
The front wheel will have to be laced different if you ever plan to put disks on the front wheel. Radial cannot be combined with hub based brakes.
Get ready to get yelled at by people about the importance of front brakes.
It's getting hot in here time to go! Good luck!