The battery pack on the old cateyes was a flat 5-pack of nickel cadmium cylindrical cells. The cells appear to be size (4/5)A. I threw my old pack out and don't have the measurements, so if anyone knows the diameter and height of the cells or the overall size of the pack, it would be helpful.
digikey.com can supply a five-pack. The part number would be P-239-F051-ND. You might want to check the size of their pack. It should fit in the cateye. The five cells in series give 6V and 1.1 amp-hour.
I ended up using a 6V, 1 amp-hour Powersonic lead acid gel cell, which is probably cheaper, and mounted it in the side panel of my bike. There is not enough room in the cateye for a gel cell.
The front of the cateye is a reflector, though it looks like a lamp.