I have no problem, or issues with redoing or restoring old promos, as I am a builder not a collector. As a fan of 1950's cars I've been pretty lucky over the years in acquiring good, unwarped promos for most years of cars. Obviously straight, unwarped cars from the earlier '50's are harder to find, but I've been pretty lucky so far in searching through EBAY listings and various vendor offerings at model shows (when there were shows to go to).
The biggest "problems I've had with some promos is finding either glass or interiors, but so far I've been able to cobble together glass using leftover pieces from kits, or using clear acetate cut to shape. I was able to acquire just about everything available from Modelhaus for 1950's cars that I needed before they shut down. I was able to get all the kits, conversion pieces, replacement parts and accessories that I needed, and have it all "stockpiled" by year for the '50's cars that I have yet to work on.
I usually add windshield wipers, inside & outside mirrors, antenna's and license plates to the cars to improve the looks and "realism" of them. I also like to add whitewall tires.
The biggest problem I have run into is trying to get rid of the infamous, dreaded, Johan "smile". That is a battle that I hope to win some day...... way down the road.