You could also respace a track hub if you could one with a narrow enough body. The chainline would probably need some adjustment in the front to.

Also as a side note here you need to run two brakes on it if you plan on ever taking it off road. To do otherwise makes you a piece of **** deserving of severe beating from anyone else who passes you on the trails. Even with brakes unless you are a very talented MTBer and know the trails you plan on riding on like the back of your hand already please just stick to ss.