Track and Field battery replacement

I've got a Track and Field and a Hyper Sports PCBs that both still have the original (and now dead) batteries on them.

I understand that these can be replaced with either a modern lithium battery or a remote battery holder and figure that this be a relatively simple job to do.

Has anyone got any tips on this, or are there instructions already in the forums here somewhere? (couldn't find anything in my searches)

Does Bob Roberts or anybody happen to sell a pre-packaged kit for something like this?