Would something like this
halogen work for you? For the price, it might be worth a try. It certainly would be easier than trying to build a rectifier and stuff it into your headlight along with a heatsink. I've messed around with that in the past and didn't care for the results. I wound up just going with a modern B & M "Classic" lamp which has all the conveniences of a modern light (including standlight). Your project looks to be very period specific, so I understand your reluctance to go with that option.