Regarding age, if it's cooked does it matter? Before you replace it you might want to charge it with a more powerful charger, trickles are not so great at restoring a dead battery as they are at maintenance. Check it close to confirm that you cannot access the cells, if you can fill them with distilled water before the charge. If you charge it with a partial plate uncovered it will ruin it. As for replacement, I am fond of interstate batteries.
I have this now as my dad is sick and will never use it again, it's from our old shop.
http://buy1.snapon.com/products/diagnostics/BC4200B.asp?partno=BC4200B