yea there are several builds in this DIY section dealing with briggs engine's including mine. belt drive probably is the easiest, quietest, and cheapest. make sure you gear it very low for take off power.
I have a Whizzer and I love it, but not the expense. I am convinced that the answer is the B&S conversion. I have only seen one that I like it is older and done so it looks the most like the Whizzer. It came out to the Long Beach ca meet at Eldorado Park, I was not able to get over to see the bikes last year, I hope they do it again. Have fun, Dave