Was going to add that in most cases the negative side to anything usually is tied to ground as well. Not always of course if the battery or component is between other components, but frequently the negative is ground. So using a meter you can tone/bell/ping the terminal connections on the mother board and find out which is Ground. In his case it was all clearly marked on the mother board and he just needed to know where + was on his replacement.
I would have gone a step further as well and actually installed a coin cell battery holder to allow easier change outs in the future. I do this with most anything I have that uses batteries. Even my NES, SNES, Genesis, etc game cartridges I install battery holders. Did the same with my MT-200 modules as well.

Midi Modules: MT-32 (OLD), MT-200, MT-300, MT-90S, MT-90U, SD-20