RE: Deep Cycle Starting Battery
First, the Group 42 battery you have is the wrong battery if you want to
be at factory spec.  Spec is a Group 41.  The 42 is physically smaller
and less reserve power.  BTW, the 42 is the only standard battery that
is physically smaller than the 41 according to the BCI official
listings.

There are small deep cycle and sealed batteries that will fit the space,
but none of the same capacity as the 42, at least not according to any
reference I've been able to come up with.