I looked a little bit into directly driving a go kart hub with a BN M1-AT gearbox. That might be worth a try if you like high speeds. Obviously packaging is a problem so again not a great all-rounder board.
So I would rather take a stab at a belt/chain drive first.
I had to pick some hub that would fit 11x6-6 tires. I also wanted to find a hub and tire/tube all on the same site.
it isnt mentioned if you can stack two of the spacers, I assume so but its not recommended for some reason.
This is what I settled with. The hub is technically a 5in wide hub with the 1in spacer but it will be fine enough to try with an XR tire. I am sure its heavier and more expensive than a more standard kart hub, but I like that the side of the rim is not super deep making it easy to attach a pulley.
I have a set of XR rails, a giant 12mm bolt that could serve as the axle, and now the wheel assembly. So next would be plan out the motor mounting and how the belt or chain fit within the frame.