For battery replacement on the clock we recommend taking it to your local jeweller as you would your own watch. 

Because the gauge is waterproof it requires special attention when it is reassembled. The gauge's back is a screw back cover that requires a special tool to remove or replace. Most jewellers can easily do this for you. 

The battery is a common one - SR626SW, UCC 377, Ray-O-Vac, RW329 or equivalent - and can be found in most camera shops and local stores.