I figured as much. Didn't think they were quite that big since it is supposed to mount to go on the driven of a 30series though
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They're not.
They're about this size of a 212 block, made of aluminum, and no more difficult to set up than a torque converter with a standalone jackshaft.
The shifter is just a lever and a cable. You can pretty much put it anywhere.
The reason big heavy karts have them is because big heavy karts are a PITA to push backwards out of a bad corner or even out of the garage. Plus it's easier to justify the cost on a big kart. Most companies that used this type of standalone FNR box were using comet, spicer, peerless, Dana, etc boxes, not Chinese copy ones you can get for under $50/ea of you buy a whole shipping container full of them.
Putting them on yard karts would have priced the karts out of their own range, and for what reason, when you can stick your leg out and wheel yourself backwards in a couple seconds?
That's why you don't see them on small light karts.
It's the same reason an economy car doesn't come with an 8 speed twin clutch paddle shift racing transmission with cryogenically treated straight cut forged gears. You wouldn't sell many Ford fiestas if they cost 65 grand.