You need a test meter to determine voltage unless you are sure it is a 6V light. If it is 6V, a 12V is not going to cut it unless you change over to a 12V system $$$. You don't say if it is a small bulb that can be replaced or a large headlight assembly similar to a car headlight, a sealed beam setup. Bulbs can be found on the internet. A specific sized sealed beam is going to be harder for sure unless it is something an old car or tractor used. Might find a reproduction small 6V running light that fits an old car.
if it is an original light and it is dim, the first thing you want to do is make sure all of your connections are good. No loose or frayed wires flopping around in the wind or where they are fastened to spade, or round connectors or spliced together. Take off the connections and clean the connection and the place where they are connected to with a wire brush or fine grit sandpaper, steel wool, something along those lines. If there is rust/corrosion in any bulb socket, it either has to be cleaned out of there, or the socket has to be replaced. Electricity has to be able to flow like a river. Bad connections are like a dam. The the electricity has to go around or through these bad areas, and voltage is lost.
71 Dodge : What you need to know is the amperage of your lighting coil ? Is their a battery in the circuit ? I test amperage with a amp meter and I am 100% positive that a small engine won't produce enough power to fully light up a 55 watt bulb. My Tecumseh engine has a dual coil a 3 amp D C coil and a 5 amp A C coil. You should supply the model number of your engine and a picture of the connector plug then it might be information in the manual to help you. Steve 
Dodge, The formula you need is: P=IxE where P is the watts and I is the amps and E is the voltage. In your case where you want to fully light a 55 watt bulb with 6 volts; your motor is going to have to put out at least 9.2 amps. Like Steve said above; your motor is probably only putting out around three amps and that's not nearly enough to fully light your 55 watt bulb. Using the above formula, if you do indeed have 6 volts and 3 amps the best you can hope for is about 18 watts. Hope this helps. Ogy
71 Dodge : What you need to know is the amperage of your lighting coil ? Is their a battery in the circuit ? I test amperage with a amp meter and I am 100% positive that a small engine won't produce enough power to fully light up a 55 watt bulb. My Tecumseh engine has a dual coil a 3 amp D C coil and a 5 amp A C coil. You should supply the model number of your engine and a picture of the connector plug then it might be information in the manual to help you. Steve
According to the formula that should only require a little over 4 amps. If your coil is putting out only three amps; you're still going to be shy of the amperage needed to light your bulb to the full 25 watt brightness. With your output of 3 amps at 6 volts I'm afraid you're limited to right around 18 watts. Ogy
