13V is a little high, I assume that's resting with the charger off? that will happen if the acid is concentrated from low water. Fill the water, use it normally, let it charge overnight, unplug charger and see what each battery has for voltage, if they're all roughly equal, then you should be able to get some life out of them yet. Depends entirely if this is something you use once a week to lift a couple times, or if you use it 40 hours a week.
27.1V float is fine if that's what you have with the charger plugged in after a full charge. It will take 28-29V for the bulk charge, and it might not get there if it's not taking amps fast enough, or there's too much resistance somewhere in the connections.
I'm not sure what to think of desulphators, I've used them, haven't done any controlled testing to tell if they work. Can't hurt? A smaller charger will charge just fine if you give it time. 24V should be easy to find. If you don't need full capacity, using two 12v marine starting batteries will work fine and be cheaper to replace even if it's a little more often. You won't go off hard surfaces, but you should be fine for typical scissor lift work. Actually the 25A should work fine with marine batteries also, I wonder if the original charger was higher amperage than 25A? or maybe that's all they could get out of a 115V outlet in those days before inverter chargers?