Looks like the same variable-diameter pulley drive system the original lawn tractor (or perhaps a snowmobile?) may have had. Also, note that he says he's driven over 7000 miles, with an avg of 115 mpg!:drool:
I like old bikes, but I have to admit I'm not real fond of those 4-carb engines. My son had one very similar to the one pictured, just a bit larger- a 1979 GS1000. That diesel conversion sounds very interesting!