I gave my answer too hastily before reading your original post thoroughly. The modern tail light will probably consume less power than the old one, leaving a tiny bit of extra power for the headlight. But the headlight is probably lousy. Also, modern lights have voltage regulation built into them, but your old headlight has none. The extra bit of power might increase the likelihood of burning out your bulb.
As @
steelbikeguy points out, a replacement bulb that is LED may not work well at all. I tried one of those in an old two-D-cell type flashlight, and the result was terrible.
B&M makes headlights that are styled like antiques, so maybe you should try one of them.