Various JUNO-60 manuals and information.
The JUNO-60 uses a soldered-in battery and over time these can leak or become bad of course. The most common method is hacking in a CR-2032 battery holder and then screwing that into the wood, however there's a much more elegant method. You can purchase a surface mount 1/3N battery socket and it fits exactly in place of the original soldered battery! And then you simply need to install a 1/3N cell. Sometimes 1/3N batteries and sockets are referred to as '1/3 N' or '1-3N'. Digikey page for the surface mount socket for reference (some synthesizer resellers will sell this sockets at a huge markup but I would suggest just buying them directly from electronics suppliers instead):
Digikey part # 36-498-ND
Manufacturer part # Keystone Electronics 498
There's a few upgrades and options for the JUNO-60; I personally would just recommend the valpower DCB-to-MIDI (USB) adapter. Yes it's very primitive and only has note on/off, but I think it allows for more thought behind patch programming to work around this limitation and doesn't dramatically change the instrument.
For a more comprehensive upgrade, including adding more LFO waveshapes, enevelope generators for the filter, polyphonic portamento, an upgraded arpeggiator etc... the JUNO-66 mod would be the way to go: