The reduction box is not really the problem. The OP was looking at putting a car engine in a plane, and to that end they are not really compatible. There will be exceptions, to prove the rule in most walks of life. I was at St Omer just before Christmas and a chap there has a Peugeot Diesel in a small Plane, I think it was a Jodel but not sure. But as a generalisation car engines are designed for a very different role.
Modern car engines use a lot of RPM to achieve the power output for the sales brochures, but most will only be turning at 3-4000 rpm at normal cruise speeds(70-90mph, don't you love those German autobahns?) Having just written that, it is interesting that the car and aircraft engine are designed to operate the majority of the time in roughly the same RPM range? I would have thought it has something to do with internal friction losses and heat build up?
Any how off on my bike to work now, which funnily enough is a Honda CBF600 with fuel injection and miles of wiring to all sorts of gadgets
Most of which are designed to meet stringent emission regs and have sod all to do with power or longevity.
Nice memories IO540 of the Escort, the Mk3 was a legend in its day, not forgetting the RS Turbo, never really whent much faster though with the turbo bolted on
Justn looked better in the rally colours.
Safe flying PG