I looked into it and it doesn’t really look all that hard.
The problem is defining the drivetrain: either non-drivetrain engines become new parts (bad idea, because every engine would need a new part) or drivetrain becomes a new part.
So drivetrain would need to become a new part.
The problem with adding a new essential part is adding it to existing vehicles, of which there are tons. Some in mods. Non-core mods would get hit the hardest because all the vehicles in them would be broken until authors added drivetrains to them manually.
Then save compatibility, but that’s easy: every vehicle from old saves would get a drivetrain on every engine for free.
Later on we could split drivetrains into more complex ones: truck drivetrain, car drivetrain, bicycle “drivetrain”. But at first it would probably be just universal, perfect drivetrain that weighs next to nothing, has no transmission loss and no limits on usage.
tl;dr Not much coding, totally doable (just not before 0.D, because feature freeze)