I would suggest replacing it with a LiPo battery unless it's a 'vintage' flashlight that has to look original. I am doing a similar exercise with a vintage lead acid glass accumulator using a LiPo single cell and a 'buck/boost' converter. Many modern LED flashlights seem to use the same method? Just a thought...maybe a photograph of the battery may help other contributors....
in the times of old, when car lead batteries were scarce in my country, people were using pulsed charging, where battery is subsequently charged and discharged, which could help do dissolve sulfuric sediments - but there are cases when it is not enough...
There are even more elaborate methods - with replacing electrolyte with something that dissolves sediments then filling with fresh electrolyte but in such a case maybe replacement battery would be simpler and cheaper?
