Ed - Check out Simsondesign.net
The just prior NC Miata model seems to be most used (it's a little bigger than older NA/NB models - including the latest ND. A beautiful Aston Martin DB4, Ferrari inspired Italia, and some more basic classic looking race cars like the Swift. No prices are given.
Also check out - Miata Based Kit Car Directory - Kit Car Directory (miatakitcar.com)
which has an MG TF listed and Lotus 7 look cars.
My guess is as mainstream automakers move away from ICE - these kits will become even more popular.