We'd have to know the wattage of the bulbs that's in the headlamp now. Typically 55W is about the max for 6V and usually any generator can keep up with that discharge rate.
P.S. Since that's an older machine, make sure that all of your wires are in good condition and delivering full voltage to the lamp. Load test your battery and if it doesn't hold specified amperage replace it. Lastly, make sure your generator is supplying sufficient amps to your battery.