A 100Ah lead-acid battery like the Leoch Adventurer AGM 100Ah only delivers about 50Ah usable at the 50 percent depth of discharge that keeps lead healthy, roughly 600 Wh. Because LiFePO4 safely uses about 80 percent of its capacity, you need only around 59Ah 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 600 Wh from your lead, and saves roughly 14 kg.
Fit: your Group 27 box is 306x173x225 mm, the lithium is 330x172x214 mm. Check the space, larger than one original box
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.