I put an LED bulb in an old fashioned flashlight, the kind that used two D cells. The results were pretty poor, because the LED directs the light forward, and the parabola assumed that the bulb would spread light evenly in all directions.

I have successfully put modern LED headlights and tail lights on old dynamos but not old dynamo hubs. Two were the sidewall (aka bottle) dynamos, and one is the kind that goes where a kickstand goes. The results are every bit as I had hoped. Mechanical drag from these dynamos is greater than with dynamo hubs, but the light it produces is excellent. The advantages are cost, weight, and ease of installation.

[MENTION=73614]rhm[/MENTION]'s point seems to be that a modern LED headlight might not start producing good light until the power (wattage) reaches a certain point. I haven't worked with those old dynamo hubs, but these old dynamos I've installed seem to produce an excess of power. How much power do the modern headlights really need before they start doing a good job?