You could go with Shimano's new 11 speed internal gear hub, it comes in a disk and I think the spacing is 135.
Spacing on a Rohloff is 135 too. Both are available with a disk and they have fantastic gear ranges.

You would have to zip tie the shifter cable on your frame, but you would have gears with no modifications and if you ever wanted to take it back to single speed all you would have to do is swap out the rear wheel.