If you take the bellhousing off a rear wheel drive (and a few front wheel drive ) gearboxes they become fairly compact, not as much to fit on your pocket, but compact enough i would think for a large go kart, or tractor.
BG civic? gearboxes are relatively small, i would be after a toyota e series gearbox because it has shifter cables rather than shifter linkage, out of an e80, e90, e100, ae92, ae86, or a small nissan rearwheel drive box, say an L20, GA, SR code engine.
The mounting and shaft work is simple, just takes time, and the great thing about front wheel dirve gearboxes is that you could plonk it between the back wheels, and run the axle off the diff.
It all can be done, it just whether you have the time and money.