How about the ability to create a stand alone engine, then have the game automatically display a list of chassis the engine will fit into?
So you are now saying that the gameflow has to be flipped front to back?
Not going to happen, though, later on, I think the engine will get a volume stat, which shows a number and that number can used to check bodies who have enginebay volume.
Body A has 290 litres of engine bay space
Body B has 245 litres of engine bay space
You made an engine which requires 272 litres of engine bay space
Obviously, it would fit in A and not in B
The main issue with that is the engines with funky dimensions and more restricted engine layouts (transverse i6 anyone? 
). Even smaller displacements won’t fit.
There has been talk of making it easier to figure out the dimensions, my main issue is that its hard to figure out which dimension pertains to what direction!
Like strop said, the main issue is that the engine bay volume doesn’t give much info.
For example, the not-CRX, the van and the '95 hatch can all fit engines longitudinally with a RWD transmission.
If you go longitudinal 4WD though, absolutely nothing fits, not even the smallest I4 possible.
Also, the not-golf, as well as the two smaller cars from the 60’s can fit pretty decent engines longitudinally, but only very small engines transversely.
Currently there is no way to tell if an engine will fit apart from trial and error, and a bit of logic in some cases (ex: 10.9L in anything but the barge.)
I think the main issue is your front suspension type, double wish bone will only allow lawnmower engines in some of the bodies.
I think that was going to be reworked a little bit; early and mid 90s civics had 2.0 with front double wishbones which if I recall correctly, cannot be recreated because of the transmission placement