Lithium battery replacement is reasonable if
having at least the storage capacity watts of older pack. This maintains runtime expectations using L/A battery.
On the other hand, if battery still takes some charge after sitting around for a year, there's chance one of those advanced 'battery reconditioning' chargers using special pulse technology can restore an abused L/A.
However, unless already having one of these healing chargers, cost to get one is maybe same to double a lithium battery replacement. And renewal is not always successful if battery refuses to charge at all.
Only found one and tried it out mostly on newer L/A batteries stored a bit too long, and seems to
mostly work as advertised.
The redipulse 6/12 model is the one I've had for a few years, and now I see a new model at higher cost.
http://www.batterystuff.com/battery-restoration/6-volt/