I'd be interested to see how that light performs with a lithium battery.
That's probably a 55W halogen bulb, which means it only draws around 4.5A. Most 18650s can supply this easily, and 3S will provide around the correct voltage, without going too high for the bulb. The voltage drop from the original battery was probably bad, and you'll likely get better performance across the life of the battery.
If you used 3S 21700 like Molicel INR-21700-M50A, you'd at least
double the original runtime, I say at least because SLA battery capacity drops quite a bit at higher currents. I'd guess at 2C you're close to halving the real capacity of the cell, making that effectively a 1.4AH, and giving you about 15 minutes of runtime. Am I close? You'll get probably an hour on 5AH 21700s.
If you're not comfortable making your own pack, you're going to pay quite a bit more but BioEnno might be a good place to start: https://www.bioennopower.com/collections/12v-series-lifepo4-batteries
Compare your dimensions, and find one that will fit. Keep in mind that whether you go with LiIon or LiFePO4, you will need a specific charger for it, don't try to charge it using the light's built-in charger. I'd remove it to be safe.