You need a 3v lithium battery, most common is a CR2032 button battery.Vertical holder - (https://www.ebay.com.au/itm/5x-CR2025-CR2032-3V-Button-Coin-Cell-Battery-Vertical-Socket-Holder-Box-Case/184279895404?hash=item2ae7f00d6c:g:i~0AAOSwHcxesTds). Not sure why it has 3 lugs/connectors, you only need 2, also getting the right spacing for the PCB holes can be problematic, you probably need to bend the lugs to fit.
One with lugs attached - (https://www.jaycar.com.au/cr2032-vc-3v-pcb-mounting-lithium-coin-battery/p/SB1762). This is what I used for one of my MP-2s, had to bend the lugs to get them to line up, again you only need 2 connectors (1 x +ve, 1 x -ve).
I haven't heard of anyone having a bad EPROM, I spose it's possible but unlikely. A factory reset should load all the EPROM programs into RAM (user) which works while the unit is powered on. IIRC you can switch between preset and user, well you can on MP-2 which the Classic is similar to (in some ways). I think maybe Preset accesses the EPROM programs, user accesses the RAM and you can modify and save the patches.