Here's my attempt at the three wheeler idea. A little different than what you have planned.
www.highmileagetrikes.blogspot.com
You'll also see some other interesting things on the site as well.
You're on the right track, rather than building from scratch like I did. automotive engines and front wheel drive are the way to go. For a glance at ultra light, look up Jory Squibb in google image search. I was just about ready to try something like his 3 wheeler, but am currently obsessed with converting my crx into HM vehicle. Jory gets 100 mph, I'm almost half way there, and I can go on the freeway in comfort and incogneeto!