What you might want to look into - rather than adapting the motor/tranny to a chassis, would be to use the entire bike, from the forks on back, ala TriMagnum. Plans for doing this are available from http://rqriley.com/
TriMagnum
Or, in the alternative, you could adapt a front-wheel drive set-up with a pancake motor (like a Subaru or Citroen) and just have the rear wheel along for the ride. One of the coolest little cars like this was sold just down the street from my house, called the Trihawk. There aren't really any plans, but they did have a 3D layout setup in their brochure which is available from a number of places on the internet. You might want to start here: http://designmassif.com/trihawk/
www.3wheelers.com/trihawk.jpg" border=0>
Trihawk
Here's an overhead view of the trihawk:

The T-Rex is a very trick little package, but it's pretty durned expensive for my tastes...And using a Harley motor - as neat as it is - isn't the best performance engine out there. Just my opinion; I like the stock Japanese cruisers alot better than the stock Harley offerings.
The way you build up the TriMagnum is pretty easy. I built one from the article in Popular Mechanics, I didn't buy the plans until much later. It's that easy to do.
In any case, one thing to remember about motorcycle kits is that they don't have a reverse gear - except for the big cruisers like the Honda Goldwing. The pancake motors will use an automobile tranny, and shouldn't be that hard to fit to a chassis.
Hope that helps out a bit.
Your pal,
Meat.
