About a decade ago I sent away for some plans for a fully enclosed trike made from a modified bicycle. I'll see if I can find them and post them here if they are no longer for sale...
As far as a tranny with reverse thats small enough...........do older atvs have reverse? I know alot of golf carts do, gas and electric plus those gas ones where you step on the gas pedal and it just starts and goes.
If you go to bentrideronline, there is a homebuilder sub-forum, and there is a lot of trike-building links in there. A reverse trike is usually called a "tadpole" trike.
If you have access to welding equipment (and somebody who knows how to use it) it is not that difficult to build a bike or a trike--if you can scrounge a couple or three junk bikes to cut up, but the transmission with reverse,,, I don't know where you'd get that. The main problem most bicycle engine kits have right now is that they are only one-speed drive. The only ready-made solution is the Staton/NuVinci setup, and it's got no reverse.
...What you will not find much info on there however is adding an engine. So you'd have to come back to here to get much help with that.
------
Just to get started, here is one site where the guy has built a lot of trikes (and bikes too): http://fleettrikes.com/
~
Thanks Doug, I am going to look at the site you sent and see what I can come up with. I just got through building my first "normal" bike and I would say that in a little while I will be ready to start working on the trike. Thanks for the link. I will let you know how things work out!