check out
factoryfive.com
they do some pretty insane kits
www.superformance.com
I've been considering doing a kit myself for a long time...
the other route to go is to get one custom welded. Drawing up something to mate to suspension/engine would be an interesting challenge, and paying to have it welded up and powdercoated shouldn't be that expensive...
Seems that one could be able to make themselves an ariel atom type rig for about 10 grand, or less, depending on how much time you could put into it. Very time consuming, and the end result is probably pretty similar to a miata/914 but still....