everyday driver did a great multipart series on this a few years ago:
https://www.youtube.com/watch?v=ifK7m-snRSY
20k buys the nicest NC miata in the world, which is the direction I would take. The german cars are faster but part of the charm for me would be a car that I am not messing with much.
ND miata would be closest to the elan in ethos, you can probably find a high milage ND for sub 20, it will still be cheaper to own than a aged german sports car.
For fun to drive factor, low weight is everything in my humble opinion. Lap times and power are inconsequential. Sound of a flat 6 or I-6 near redline is a treat that would go missing in the miata though