A 110Ah lead-acid battery like the Leoch Adventurer AGM 110Ah only delivers about 55Ah usable at the 50 percent depth of discharge that keeps lead healthy, roughly 660 Wh. Because LiFePO4 safely uses about 80 percent of its capacity, you need only around 65Ah of lithium to match it, at a fraction of the weight.
Best-value lithium replacement
LiTime 12V 100Ah
Gives 1,024 Wh usable against about 660 Wh from your lead, and saves roughly 19 kg.
Fit: your Group 31 box is 330x173x240 mm, the lithium is 330x172x214 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.