This thought continues to intrigue me so I spent a few minutes on Glenn's registry.
I went through the first two pages of early cars that included photos and categorized them based on these criteria:
Stock - no apparent mods and looked to be maintained or re-done with a nod towards originality
Light mods - Narrow body cars with no apparent desire to remain stock (changed mirrors, wheels, valences etc)
Heavy mods - Mostly wide body cars; 471 mods or GT look cars
Race - obvious race cars
Here are the numbers -
97 foto's viewed; 33 Stock looking cars (34%); 17 Light Mods (17.5%); 30 Heavy Mods (31%); 17 Race Cars (17.5%).
So going back to my original guesses - 1700 imported with 30% "gone" leaves 1290.
Applying the results of the two page foto survey:
34% of 1290 "stock" = 439
17.5% of 1290 "lightly modded" = 226
31% of 1290 Heavily Modded = 398
17.5% of 1290 race cars = 226
So by this quick survey, there are around 665 cars in original or somewhat near original shape.
I was completely taken by surprise at the percentage of modded/flared/race cars!! Nearly 50% of the population.
A valid criteria for "original" could be matching numbers and few mods. My guess might be that half of the 439 "stock" cars have matching numbers with largely stock motors. So from that I would guess that the number of original or near original cars is maybe 220.
Close enough for 914world work

This 220 number would seem to me to jive with the number of "for sale" or known "original" cars that we see out there in the public domain. At that number, and with the amount of dealers and collectors and enthusiasts, the $65,000.00 - $100,000.00 prices for nice cars seem "supportable".
I think the biggest place for error is in the assumption of 30% gone. This could be a lot higher. Any other insights out there??