The oil becomes its hottest at the crank a rod journals. Then it is cooled by the engine cases and the oil pan before it gets to the scavenge pump (CB750). The oil tank gets the hot oil next and it's outer surface area cools it some more. The oil pump draws this oil from the tank and pushes it to the oil filter where the canister cooling fins remove more heat before feeding the oil galley feeding the main and rod journals.
Note the physics of heat transfer states that it occurs most rapidly when the source and destination have the largest differential. In other words, a 90 degree heat sink will draw heat faster from a 300 degree source than a 200 degree source given the thermal transfer efficiency remains the same. So, place your heat exchanger closest to where the oil is heated the highest, as that will cool the oil the most rapidly using the same size surface area exchanger and the same air flow.
IMO, you only need an oil cooler if the oil is being heated above accepted limits (300F or above depending on your oil choice). And, it is the motor cases/cooling fins that keep the oil within those limits.
The SOHC4 is primarily an AIR cooled motor. And while the oil does carry some heat away to external cooling devices, it is only a very small part of the needed heat exchange. The air is where the heat is absorbed, then carried away by its movement/replacement.
Have you thought about using your computer fans to keep air moving over the engine fins? If you keep the motor cool the oil will also stay cool.
Consider that the oil may remove 10% (WAG) of the engine heat. Even if you double the oil to heat exchanger efficiency, the overall heat exchange of the motor has had minimal impact with the addition of an oil cooler.
Conversely, over cooled oil does not lube as well as correct temperature oil. Beware of designing a system that ONLY operates properly in hot whether under high power settings and minimal airflow over it, as at other times the oil won't do it's job as well as it should.
Cheers,
