It's an air cooled motor.
It is cooled by the outside surface area transferring heat into the air moving past it. This is an air exchange process.
Oil coolers work on the exact same principle. No air exchange, no cooling.
Further, an oil cooler is NOT a substitute for the motor's primary cooling device. It is to help the oil survive high pressures in the crank/rod journals (under high power loadings) which raise oil heat levels. Any cooling effect on the motor is, at best, a tertiary contribution. An oil cooler's surface area compared to the motor's 'as built' is almost laughable.
Honda sized the engine's cooling apparatus for the power density of the engine, as over-cooling it as harmful as under-cooling. What have you changed to make it different?
If you are doing low or no speeds to make air move past the motor, you need an air moving fan to force air past the engine's incorporated cooling fins/outside surface.
If I were to operate my SOHC4 in a slow moving parade in hot weather, I'd mount a couple of computer fans to force air over the motor.
Cheers,
