The weight is almost entirely immaterial.
Around here roads go down and up besides miles of long sort of level stretches
We can Google and use the standard formulii to get close:
This document gives a Cd for a semi-upright motorcyclist (60 deg leg angle) and motorcycle as 0.78, and the approximate frontal area as 0.68 M^2.
Fd = Cd 1/2 ρ v^2 A
Given standard conditions and your specified 75 MPH (121 km/hr)
Fd = 0.78 1/2 (1.2 kg/m3) ((121 km/h) (1000 m/km) / (3600 s/h))2 ( 0..68 m2)
So, our drag force = 360 Newtons
To get the power required to achieve this:
Power = 360 N (121 k/h) (1000 m/km) (1/3600 h/s)
- 12,100 watts, or 16.3 HP.
500cc can do that "all day", so could a Honda 305. You are arguing theory against experience
2 x 16.3HP = 32.6 HP, well toward the upper end for cruise (75% power) of the OP "30 to 40 HP" engine requirement.
2 x 500cc = 1000 cc = 61 in^3.
So geared 60 cu in vs 100 cu in direct drive?
Once gearing is chosen, there can often be great efficiency gains over direct HP if even longer props can be used than the competitive HP direct drive unit in a given flight application. E.g. a 175HP GO300 with 84" prop has performance advantage over 180HP O360 with 76" prop in same 172 airfame.
(long straw argument)........Yes, we can add fin area and more effective baffling, etc. But there are diminishing returns. The issue is the finite heat flux ability of aluminum (or any solid). In our engine, heat is being added to the inner surface of the combustion chamber and piston. That heat needs to flow out to the fins where it can be exchanged. This flow occurs at a rate that depends on the cross sectional area of the aluminum and the temperature gradient. Extra fin area helps, but the physical limits of the cross sectional area of the aluminum eventually imposes a limit. When the temps inside reach about 500F, your head will live a very short life.
A thought experiment: We buy a roll of aluminum foil and unroll the whole thing. We set up a fan to blow air over this tremendous expanse of foil. If that foil was at 300F, we could easily shed thousands of watts/hr to the passing air and the foil would never get hotter. But what will happen if we put a little flame (500 watts?) on a corner of that foil? It will rapidly exceed the melting point of aluminum. Sure, the expanse of foil was capable of shedding thousands of watts of heat, but the aluminum in the immediate vicinity of the local heat application couldn't move it away as fast as it was being added, so it got hotter until it melted. We count on this same thing if we are sweating a connection on a long pipe: we add heat with the torch faster than the copper pipe can carry it away, and eventually it gets hot enough to melt solder. The long pipe easily has enough surface area to dissipate the heat from our torch to the surrounding air, but because the copper can't transport it away fast enough, our connection gets very hot. And copper moves heat much better than our aluminum head does.
If an engine can run for an hour in a given regime, the cylinder head is not going to melt after 2 hours.
What you seem to be arguing is that you have not experienced it and you don't understand it, so it can't be!
But examples of the engines you say can't exist are out there running steadily.
Airplane engine designers are smart. If they could add more fins to their air cooled creations and thus reliably make .75hp/cu inch, they would do it. Gearing/PSRUs can let us turn a more efficient prop for a given HP output, but it does nothing to improve the thermal transfer issues that fundamentally constrain air cooled heads and cylinders. Where are all these geared air cooled airplane engines?
Pobjoy did it in the 30's and the engines were fairly widely used, for the time.
AFA as the rest of your Q: Most of the engines used in WW2 were geared; as were the transport/commercial piston engines that followed. All turbo props are. There were plenty of successful small GA geared engines built after the war. PSRU's are somewhat analagous to turbo charging - works great. but economically only suits a really small percentage of operators.
Your concerns about heat issues are correct, but you are not recognizing that powerplants that exceed your personal imagination or calculation limits exist and have existed for most of a century. To try to make the points you want to make you set up straw arguments and tiptoe around the concept that an efficient engine by definition converts more fuel into torque and less into heat. OHV vs flathead, as a very early evolution. Compression ignition vs spark ignition going further. Other types & combustion chamber configurations somewhere in between.
I'm done, though.
You keep trying to argue that things that exist or existed don't, or taking things in directions that i have not necessarily argued against as specific cases.
To repeat, since aviation with IC engines began, design consideration has been whether to go simplicity & design a large enough displacement to provide the necessary HP at a specified relatively low direct rpm for propellor efficiency; or whether to decrease physical size and often weight by designing a "high" rpm engine and getting the propellor rpm through PSRU.
You might prefer a given configuration,and me another.
You can't argue that the choices don't exist nor that both historically have provided solutions.