Not my field but the other difference is 1500 hp at 2,400 rpm. The low max rpm is because it needs to match the propeller design speed and needs to be efficient at near full power.
These engines were, in fact, geared down, as was common (if not ubiquitous) in large piston aero engines. This video shows an epicyclic reduction gear inside the bell-like case on the front.
IIRC, piston speed was an issue in how fast these engines could run, as, for given RPM, the piston speed is proportional to the stroke. And, for reliability and the corresponding safety reasons, aero engines are more conservatively designed than most car engines.
If you're really interested in the topic look up the YouTube channel Greg's Planes and Automobiles. In addition to a deep dive series on the P-47 just a few weeks ago he made a video about late WW2 "super prop" fighters. They were the pinnacle of piston engine fighter performance, but they were all canceled because they weren't needed to win the war and everyone could see that jets were the future. We'd probably struggle to beat them even with modern technology TBH.
Yeah it’s cool to think about how some technology might have developed if the economics didn’t make the evolutionary branch obsolete. I think that’s the appeal of steampunk fiction etc.
Gearing adds a lot of weight, and soaks up some power. Additionally, that weight tends to be at the front of the engine, so it can mess with the CG. You ideally want an engine that produces good power at rotational speeds that keep the propeller tips moving sub-sonic.
At least, that's what I found in brief investigations long ago when I was interested in the idea of a wankel rotary powered plane.
Gearing adds weight and complexity, and complexity brings unreliability. Best avoided where possible.
Putting car engines into recreational aircraft has been popular over the years, but never really 'took off', with high failure rates from being run at consistent power levels way past their design goals. The best conversions are high displacement naturally aspirated engines that end up looking remarkably like the existing Lycoming/Continental aviation engine installations.
Not maxed out, neither in terms of size nor in terms of wartime-only tradeoffs along the lines of "it's ok when two engines blow up due to low error margins if the performance gains they enable allow four boys more to return", but the RED A3 hn-famous from driving the Otto Celera is a modern aircraft piston engine. Apparently the only one, if you ignore the occasional automotive adaption.
This whole discussion makes me think what would a modern maxed out aircraft piston engine be like!