What is the cooling air exhaust velocity for the piston engine of a propeller driven aircraft?
There are cases, as in the P-51 Mustang, where the cooling air adds to the THRUST of the aircraft as it is heated and expelled out the back of the air vent, much like a jet.  This actually added a few knots its speed.  

There is always an issue of cooling as one scales up a fuel burning engine.  Towards the end of their time, piston engines for aircraft had grown exponentially in power in the race for performance.  This lead to major issues in cooling of engines such as the "corn cob" R-4360 Wasp Major.  Turbo or Super chargers also unavoidably raise the temperature of incoming air by compressing it.

So to estimate the velocity of the cooling airstream, one might begin a study of the principles of a jet engine.  Since no mass is added to the cooling stream and there is no compressor (as in a turbo jet, although prop wash may give some effect) the only considerations are ram pressure and thermal expansion of the cooling air.  Forward speed of the aircraft will be a major factor.

Study of the P-51 design may be a good place to start.