One place to start is 3-wheelers.com
(http://www.3wheelers.com/enter.html).   Although they appear to be out
of England, they have an extensive list of manufactures.

The world of three-wheeled vehicles is … unconventional, to say the
least.  In the little I have looked around, the three-wheel folks prefer
to call their vehicles “cars”, except when it comes time for license and
permits.  They range all the way from Briggs & Stratton to 5.7 Liter
V-8s that can reach 60-mph in less than 4 seconds.

For a ticket to the Far Side, Google “three wheel”.

There are manufactures that make bell housing adapters to join 13Bs and
VW transaxles.  A thorough survey would include the off-road “buggy”
people who seem to know more about VWs than a good many dealers.

Have fun, but be forewarned that much of what you will see makes defying
gravity in flying machines seem a very modest proposal.

Grant