Buy any MC battery that there is room for on the bike but beware of polarity when connecting the wires.

If you want the same capacity check the old one for how many Ah (standard 4Ah) and buy a new one with the same specs.

Thats all there is to it, no need to buy exactly the same partnumber.
_________________
Poul
"If Life Gets Boring, Risk It!"
MC21SE, KISS box
MC21SP, HRC box, racing loom