Material expansion rates haven't changed. Aluminum still expands at twice the rate of steel, and unless you make the piston and cylinder both steel, or both aluminum, you have to have large clearances in air cooled engines. Steel pistons are far too heavy, and aluminum cylinders have been proven disastrous in cars in the 1960s and '70s already. And they were liquid-cooled. Air-cooling means much higher operating temps, and aluminum really doesn't do so well there. So what new metals would we have that would change all that?
Decompression valves to drain the oil are just another band-aid to fix a problem we really don't need. More stuff to maintain and fail and add cost.
172s. They use opposed engines because opposed engines, developed first in the late 1930s, made far more sense. Right up until the Cessna 180 replaced the Cessna 190/195 in 1953, some light airplanes were still using radials. The opposed engine offered far less drag and burned a lot less oil. It was more easily muffled. And easier to see over. Radials had better power-to-weight ratios, and that was about the only advantage. A really short crankshaft will do that. That power made sense in fighters and bombers, not in light airplanes, which didn't have to win a war.
About 40 years ago a Canadian company developed a large V-8 aircraft engine called the Orenda. It was aimed at airplanes like the Beaver and Otter, as well as, believe it or not, the King Air. It didn't pan out, for several reasons but one company is still fooling with it, probably in cropsprayers. The Beaver/Otter benefited far more from the turboprop conversions.
Orenda OE600 - Wikipedia
There are big opposed engines. Continental has a 360-HP version of their TSIO-550. Lycoming has the 400-HP IO-720. Very expensive, just as a radial would be.
As you say, new engine development is prohibitively expensive, even without the significant threats of outlawing internal combustion engines or the elimination of fossil fuels. Nobody is about to put money on such stuff now.