I wouldn t like to cross a mountain range in the single
Statistically and practically, you are far more likely to die doing this by icing up (because you don't have the operating ceiling, so got stuck in IMC) and plummetting and hitting a mountain because the 0C level is below the terrain so you never got a chance to thaw out, than through a straight engine failure.
Enroute IFR flight is (or should be, unless you have balls of solid brass) done in
VMC, and (especially in a non-turbocharged plane) the engine is running at quite a low power and thus under little stress. When I cross the Alps at say FL180 the engine is probably making only 40% power, and is cool as a cucumber. FL250 is the province of turbos but even then the power output should not be high; FL250 is the ceiling for most turbo pistons.