Bill's right. Back when I painted the real thing I used 3M Pink Fill-n-Glaze. https://www.summitracing.com/parts/tes-05990?seid=srese1&cm_mmc=pla-google-_-shopping-_-srese1-_-3m&gclid=CjwKCAjw4KvPBRBeEiwAIqCB-RuXBV_mZt-aWvehJu32PkkUx2ikJ7O7r2s5tUgz_fBMLhAmnkyMFxoChnAQAvD_BwE
I just ran across something on the web. A fellow was customizing an old promo, and tried removing some paint with 91% Rubbing Alcohol. It softened up the plastic, and then it hardened up again after being removed from the alcohol. This could be a possible solution to promos that aren't a complete mess.
Here's a pic of the exhibition car. Smaller front wheels, roll bar, wheelie bars and a chute. Also, here's a link to the car on H.A.M.B. perhaps that might help you. https://www.jalopyjournal.com/forum/threads/boothill-express-where-is-it-now.765383/