What does that have to do with the drivetrain of the car? That would be due to things like better hood and road design, better enforcement of speed limits, and since you cited a stat that includes drivers as well pedestrians over forty years the move to requing roll over protection, expansion of airbag systems, more available crash modeling, and of course the near worldwide mandate that cars include seatbelts.
Most of this is either structural and thusly uneffected by a drive train conversion, or governmental.
The biggest dangers involved with a drivetrain conversion are going to be the same as any major home automotive work, namely something heavy fall or slips during work, followed by getting fingers/hair/clothing caught in moving parts, etc…
None of this is going to endanger the public, at least not to the extent that it can compete with the chance of getting distracted driving and plowing through a pedestrian or right hooking a cyclist.
Note, since the 80s the vast, vast majority of piston driven aircraft engines have been able to operate on unleaded fuel. We know this because for decades GA pilots have been filling out the paperwork for an experimental fuel variance and then running these engines unmodified on the cheaper unleaded they got from the gas station down the street without any apparent issue or rise in engine maintenance/failures among pilots that do this. The main hurdles being the necessary and not insignificant paperwork as well as concern over insurance rates.
From my understanding there was a problem with one series of engine in the seventies that was suspected to be due to unleaded fuel among the more modern product line of a major manufacturer, and while the engine was modified to fix it neither Lycoming nor Continental, the two primary piston engine manufacturers who make up the vast majority of the market, saw significant pressure to drop the official recommendation for unleaded until relatively recently.
Since the US finally started to get serious about phasing out leaded avgas in the 2010s, and the aditude of its been fine so far so why risk any change has run up against said pressure, both have to my knowledge dropped the requirement retroactively with no modification necessary for the majority of their historical and current product line.
You might need to re-engine or more likely just get an exemption for flying history aircraft, but the benefit to the hundreds of thousands that live near GA airports in terms of reduced damage to children’s nervous systems far outweighs the nebulous cost of switching the default form of avgas.