Try to ID/locate a cast part of the case that "seems" to
be the =final= , case assembly 'cover' .
mite 'snap' in ... mite be glued in .
In any event,
try =not= to cut into the 'cells' them selves ...
a judgement 'call' as "to" where to cut.
Test/replace dead/near_dead cells with equiv amp_hr "life" cells .
Mite not be a bad idea to replace 'em all.
Commercial replacements aughta be available ,
probably solder 'tab' ...
needless to say, get the terminal "connections" correct.