I gather Andy's wanting to try the construction of a small-source LED version.
"Converters" were available (but, I suspect) may have repositioned the filaments ...
You can, however, get BSF bulbs already in halogen. On my Bond Mk G, I wanted something brighter than the 24/24 W originals, so tried some Indian halogen 35/35 W ones, from eBay, and they were ... well... Hopeless. Worse than the originals as a light beam, as the filaments were - again - not where they were supposed to be.
I then went the paymore route, and got some from
http://www.norbsa02.freeuk.com and when I used those this summer, in the dark depths of the fields surrounding the Dutch Rally site, they were
excellent!
(With other, sidelight, etc, bulbs moved over to LEDs, the total load on the Dynastart is back to approximately the original total, and the car runs a slight charge with the 35W headlights on.)