I don't know about a one stop shop for the whole shebang, but TPC sells the gusset kits at a reasonable price.
http://tpctrikes.com/tpc_trikes_new_102.htm
You could find any good fabrication/welding business that could weld them in. Maybe even find one within driving distance.
Before proceeding, you really should have the frame media blasted and inspected for more cracks or rust holes. Poorly maintained and stored trikes usually have frame rot. The gusset kits could be the least concern.
The story of three wheels and a man...