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Power = torque * revs
If one chooses the correct units then there is no other multiplier.
Most people measure power in HP but often it is in kW.
Most people measure rotational speed of engines in RPM. One could use radians per second, too.
As for torque, NM, lb-ft, various other things.
All these different units require different fudge factors, but nothing changes the fact that power=torque*rotational speed. A gearbox just trades torque for revs.
Too many comparisons here aren't applicable. Motorcycle engines are terribly inefficient - all those tiny pistons banging away at 14000rpm. I've done my time on 2 wheels, too. Most of today's "superbikes" average about 25-30mpg, which is appalling given the low weight.
Most car engines spend most of their time developing very little power - if they spent most of their time at say 65% of max rated power (that's about 120mph for a car which can do 140mph) I am sure the reliability profile would be very different.
Even stationary engines like generator sets don't normally do 65% power continuously.
I've had several "engine failures" on cars due to coolant loss; even one on a Toyota. The plumbing on an aircraft engine would have to be very good indeed for this failure mechanism to not be the prime feature in forced landing statistics. It can be done because it's already done with oil, but oil is noncorrosive and the flow rate is a lot less that it is on a water-cooled engine.
As for fuel efficiency, I haven't seen any figures yet suggesting that an old Lyco is any less efficient (in cruise) than any other Avgas engine, no matter how recent and no matter what electronic controls it has. One is indeed stuck with thermodynamics....