My Batavus doesn't have sealed beams, but a replaceable 1057 6V 21W bulb, which, of course, is too weak, especially at low speeds, so here's what I did:
I took a standard 1057 bulb, gutted it, then wired in a standard G4 two-pin base, filling the base with epoxy after wiring it. Then I got some G4 halogen bulbs, 6V 10W. The new assembly fits quite easily in place of the standard 1057, and the brightness is much improved.
Easy to change burned out bulbs, too, plus they're easier to find than 6V 1057's. I've been using this for almost a year now, and it seems pretty reliable. Kind of a pain to make, but worth it in the long run.