Well I came up with a neat Idea I think, sorry if someone had it first. Since I was thinking grading the car should be simple, I believe it should come down to the parts involved and space. It would break down a little like this.
Points out of 100 for space for engine, the biggest engine bay possible 100 and the smallest 50 ( though this won’t affect final rating might help give a idea how big a engine can be sat in it.)
Points for safety features, like glass, lights, etc, added together out of 100 again.
Points for size of cargo bay which would be swapable if the engine was a RR typed, which means the front is used and FR would mean regular trunk, where MR would be a mix of both.
Points for suspension out of 100, if it is a leaf spring in the 2000’s likely not going to go anywhere.
Points for how fast it can be built and materials it is built from, persay you can have a REALLY fast built car, and score a 50 on it, cause it is cheap / easy to build and score a 100 on it if you devise a way to build it well and pick the right materials to make it from.
Passengers, for that type of vehicle, per say if it was a van you would score low if it could only seat 6 people when others can offer 8 etc which means it would have to have a set category before the car was even built.
As for that, that to me is just the car, now for the engine points it might be a little hard but I’d do it off of max HP per L or KW… like if you managed X amount of KW which was the max for X liters, you would score 100 for the performance aspect, which then all the other already set in place things would be scored, like emissions etc. And compiled into the score subtracting or adding because a vehicle producing max HP per liter isn’t going to last a long time and should get its justly deserved points.
I view my idea as a good point, but since I’m tired and to lazy to check to see if it was devised anywhere else I’ll leave it here, thanks for reading that wall of text too guys! 
Hope my ideas help
