Every now and then one of these pops up for sale, a Cushman scooter with a Harley engine. This one is a 1960 Cushman Eagle with a 1970 Sportster 883 engine. We saw one about 5 years ago with a 1200cc swap, but I think the 883 will do just fine.
It's kick start only with a magneto ignition and generator for lights. The seller mentions the popular Briggs and Stratton Vanguard conversion which is also a V-Twin, and there's one of those for sale, too, but if you're doing the V-Twin swap, why not go all the way?
I really like these combinations. Don't know what they would be like to ride, but I think you would get your money's worth in fun.
Link: Cushman Harley on eBay