Batterie will not hold enough charge. Mountfield Brigs & Stratten 1438 MSD?

I've been having to charge the batterie to start my mower , successfully up to now, but i presume the batterie wont hold enough amps now to power the starter? The mower still wont start when i use the jump leads. The starter turns the engine over though with the added boost.
Details : Mountfield Brigs & Stratten 1438 MSD
Batterie Fiam F19-12
Measurements 7.25ins long
6.75ins high
3.50ins width
Sealed Batterie with flat Terminals.
Where can i get a compatable replacement batterie and what type should it be ? As nobody has heard of Fiam Batteries.
Thanks for your help in this way, John