Please lend some assistance

I'm workingon getting my dad's old bicycle working again. I would say it was manufactured 93 to 94 maybe.

I'm having issuess with the bike battery to stay in the case. I was able to get the battery cover off the original computer. There were no corrosion issues so that's a win. I'm thinking it's just a low battery. Does anyone know what make this battery is? How to get the battery to stay in the bike computer? Any suggestions or point me in the right direction would be great thanks Chris.