Hope we never see a OHC RE. It probably wouldn't look anythig like a vintage limey. If 'modern' things like an OHC is stuck on the top of an air cooled engine the situation changes when it comes to head cooling. An OHV push rod engine can get along reasonably well because the 'rocker boxes' don't mask so much of the top surface of the head. Not so with the OHC. The top of the air cooled head is then badly compromised in regards to cooling. One of two things must then be resorted to if the engine is to regain reliability. Copious amounts of oil, which usually requires a second seperate oil pump, and a oversized oil cooler. Is that acceptable considering the small practical gain the OHC will supply. Or water cooling. Water cooling adds pump, radiator, hoses, jacketed barrels and other items that increase the CG heigth and overall weight and affect reliability due to mechanical complexity and further increases cost. In my humble opinion we'd end up with a ugly, heavy, expensive, poor handling, and unrecognizeable bike.
As far as unit construction is concerned the only advantage is some weight saveing, at the expense of greatly increasing the dificulties when repairing the machine. Now that there are very reliable and cheap primary belt drives available the reason for unit construction is probably more related to cost of manufacture and assembly ease.