Jim;

I have to ask if you have cleaned and tightened and assured lowest resistance of all of the electrical connections involved first before asking this...on 6V systems, were there is not much voltage to begin with, and certainly none extra to loose across poor connection, this is crucial!

....that will go a long way toward making the oe 6V lamps as bright as possible...and even the 6V lamps were pretty decent in brightness if they get all of the voltage, but if you are still not happy, then you can think about increasing the lamp wattage...many of these are available...but beware: More Wattage means more waste Heat for any other parts nearby (think damaged plastic or wire insulation)...or you could make up some LED based equivalents...nice thing about these is they done put out nearly as much waste heat as incandescents...but they would need to be made up...

Cheers