Its not nessesary for an engine to have 2 heads in order to be DOHC... an inline straight engine can also be dohc. DOHC simply means much fine tuned valve timings for optimum compression. An inline 4 cylinder engine with 4 valves per cylinder will require a DOHC engine most probably.
Please dont drag piaggio innn.... We are not talking about Pakistan only...!
A push rod engine can support 2 valves per cylinder, the engine cant rev very high and cant produce high compression.
Performance wise OHV technology lacks behind OHC and DOHC.
Speacially DOHC does wonders where the compression can be icreased to new levels by adding more valves per cylinder and pumping much more power out of the same cc engine... hence OHC technology does rules!
However Yes in pakistan, our beloved CG125 and piaggo too are 2 examples of fine running OHV engines. (again that does not mean that they were tested against fine quality OHCs! Best OHCs we get here are very pathetic in fit and finish.... For a real comparison put GS125 old model vs an equivallent model of Cg125!)
Now take a look at OHV engine...
You have not got much control over valve timing.. it is completely fixed!
vavle timing is a function of camshaft pushing the pushrods to open/close the vavles...
Timing chain provides abaility of tuning the timing more precisely...
Supports more valves, supports variable valve timing, more compression, more power, more RPMs!