I assume you're building in 1/24 or 1/25, I've never seen a large scale VW kit. I always have my eyes peeled for car-scale motorcycle kits, for diarama and kitbashing purposes, and they are frustratingly rare! Probably even rarer for you since I buy every one I find.![Whistling [:-^]](/emoticons/icon_smile_whistling.gif)
There are some dirt bikes that come with pickup kits, and I have a couple Airfix Suzuki race bikes, but I'm thinking you need Harley parts if your doing an old school chopper trike. I found a couple 1/24 diecast harleys from Maisto you could probably rob a few bits from that for your front end if you can find one.
I built a Beetle trike quite a few years back, I was going for a different look, though, and ended up using 1/32 aircraft landing gear parts and a 1/35 desert army truck tire on the front end. Right now I'm working on a Honda Civic trike, front wheel drive and one wheel in back.