Since you can't post pictures until you have 10 posts, here are two versions of the bike – woman's 1959 and men's undated bike.
You can see the battery pack located below the seat. I couldn't be sure how many D cells would fit in that tube but that will determine the voltage you need to supply to the light if you wish to retain the old incandescent bulb type. It's probably a screw base bulb. If it were the bayonet type without screws, you could just put in a replacement LED bulb which will work on voltages from 5 to 24 volt for about $8 from a US seller on eBay
https://www.ebay.com/itm/224590569022
You can get one in 3, 4.5, or 6V versions from an overseas seller for $3.
https://www.ebay.com/itm/324511600424
This would allow you to use the original battery pack as is with the right voltage bulb. Screw base bulbs are much harder to find. Obviously with the “universal voltage” bulb listed first, you could run it on two 18650 li-ion batteries in series. I'm not sure where you can get a battery pack for that but you can buy a pack that holds four 18650 batteries and delivers 8.4V.
The dimensions for a standard D cell battery are 1+3/8 diameter and the length is 2.5 inches so this is probably 4.5V since 3 D cells would be 7.5 inches long.