Are you sure that the oil needs to be cooled? You can search for the posting where a thermometer was installed into the oil fill port to actually measure the oil temperature.
Engine oil temperature of 180° to 220° is "normally warm." When the oil gets up to 240° or higher is of concern. Even at 200° to 220°, the first thing I'd do would be up upgrade to 20W-50 motorcycle oil. Even better would be xW-50 synthetic oil.
As said above, both the engine and the oil cooler need air flow. Why not install a 12v fan activated by a
Klixon type thermostatic switch in the head. It could just blow over the head when it reached a high temperature, or if you really want a project, blow over the oil cooler when it exceeded 220°...providing you can route the oil through the cooler so it does not rob any oil from the engine.