A 80Ah lead-acid battery like the Exide Equipment Gel ES900 only delivers about 40Ah usable at the 50 percent depth of discharge that keeps lead healthy, roughly 480 Wh. Because LiFePO4 safely uses about 80 percent of its capacity, you need only around 47Ah of lithium to match it, at a fraction of the weight.
Best-value lithium replacement
ECO-WORTHY 12V 50Ah (Bluetooth)
Gives 512 Wh usable against about 480 Wh from your lead, and saves roughly 14 kg.
Fit: your Group 24 box is 260x173x225 mm, the lithium is 229x138x208 mm. Fits the original tray
Renogy 12V 100Ah Mini DuoHeat (Self-Heating)
Before you swap
A lithium battery that fits the tray is not automatically plug and play. Check your charger profile, fit a DC-DC charger if you charge from an alternator, set any solar MPPT to lithium, review fusing and cable, add a shunt monitor, and never mix lithium with lead. For your setup see the marine, campervan and off-grid guides.