This is simply a halogen broom handle type light. It uses a MR16 12V bulb. You could use a 10W, 20W or probably up to 50W bulb. I would measure the battery compartment (space between the ribs) and compare it with 12V SLA (sealed lead acid) battery specifications on line. You'll need a pig-tail from some automotive supply place like NAPA or AutoZone. It will look exactly like the one on the end of the wire in the picture in the battery compartment. You will need to get some lugs on the end of the pigtail to match the battery terminals.
I would get the highest amp-hour rating battery you can afford that will fit in the compartment (probably 12a-h.) You'll also need a charger. An automotive charger will work fine.
Charge it only in a well ventilated area…like outside with the top off of the container.
I agree that it will need to have new o-rings in the wire sealing glands and the lens.
Do a test submerge before going on an actual dive.
The higher wattage you select for your bulb will give you more light, but will also shorten the length of the burn before recharge.
If you don't want to do any of that, PM me, I will give you my address and I will take it off your hands. I will do all the work for you and you can bid on it on eBay.