I am not sure if this would work, but it is used to convert old bottom brackets that took 1 piece cranks and converted them to a bottom bracket that used square taper 3 piece cranks. do a google search for "SUNLITE BB Conversion Kit"
in addition to what has been listed above you can never have to many clamps (spring, C-clamp, Adjustable)
pliers/ vice grips are not absolutely necessary but nice to have for holding smaller parts for welding grinding
hammer for persuading parts into position
here are two links to DIY seats
Barnett Williams Style Seat (bent conduit and mesh)
anATOMic seat (bent plywood)
I have made both, the plywood seems to be sportier as far as looks and the bent conduit seemed more comfortable to me.
Jon, have you thought about doing "Crossed Dual Drag Link Steering" like some of the Green Speed trikes. here is a link that talks about different designs http://www.hellbentcycles.com/Trike%20Design%20101%20part-2.pdf
the two links below are how I set up crossed link steering.
you can still access the site using this site https://web.archive.org/ the last link that was uploaded seems to be from 27 Jan 2014 https://web.archive.org/web/20140127034452/http://www.fleettrikes.com/