With no regulator, the only thing to control the voltage is the load. That would include the battery and lights, vs the rpm of the generator. It will never produce much at idle, nothing compared to an alternator with an electronic regulator.
The question would be how much voltage does it produce with the engine above 1500 rpm and at maximum cruise speed. These systems really dependent on the battery to moderate the system. The battery provides power at low engine speeds and absorbs the excess at high speeds to prevent over voltage. The gen is likely only rated for about 20 amps which will not hurt the battery as long as it is not at that level 24/7 like a battery charger might do.
If the gen puts out 7 to 7.5 at high speed, that's about all you can expect. An inspection of the brushes and commutator will tell you if any work is in order.
I don't know of any way to retrofit a regulator. You could replace the gen with a regulator controlled model but it will also not put out much at idle. Low speed output is built in, the regulator only stops over-voltage at high speed.
The physics of a generator or alternator depend on speed for output, an alternator runs at about 3 times engine speed (and can normally run safely at 10 to 12,000 rpm) as it does not have a commutator which can be destroyed by speed. Generators typically are belted for about 1 to 1.5 times engine speed on a slower engine.
Make sure your pulleys are not worn and the belt is in good condition.
