Hi,
use the search function for 'Battery', you will find all the info you need.

If you are going to use the original cover then the size of the battery is important, plus the battery needs to have the plus pole located on the left, you will also need pole adapters available in ebay, IIRC, in the boat accessories section.
If you mod the cables, and remove the cover, then you need a power switch in the circuit.
The other alternative is to fit one of Chas's power cable sets,
includes a power switch. Send him a PM.
Just one small point, don't select an oversized battery, the battery space is very limited.
Just found this info in my files. The 063 battery has been recommended (correct size) by a few members.Edit in red.