Hi Greg, If it helps, I have attached a pic. of the 2 types of 36D motors that Cox offered in their 1/24 sports car range.
Most (95%) of all their cars came with the TTX250 motor pictured on the left,......while the more rare "Team modified" kits were supplied with the "Nascar" 3500 motor on the right. The Nascar motors can be distinguished by the oval opening in the can, the shorter (appearing) can with its rounded can end protrusion, and the rounded top/bottom of the endbell.
If your chassis has a brass "U" shaped clip to secure the motor...the "correct" motor is the TTX250,.....if your car came with a separate stamped aluminum motor bracket, the "Nascar" is the historically correct motor........if you are at all concerned about being "correct".
Despite the "Nascar" looking shorter, the outer dimensions of both are the same, and, either motor will fit in your car.......and of the several I have owned/tested/raced, they perform similarly, although the "Nascar" was touted to be a better performer.
The 2 examples on the top of the picture are NOS examples, while the 2 on the bottom have been extensively modified/rebuilt for vintage racing.
As previously mentioned, these motors in new condition are quite easily obtained.