A 140Ah lead-acid battery like the Exide Equipment Gel ES1600 only delivers about 70Ah usable at the 50 percent depth of discharge that keeps lead healthy, roughly 840 Wh. Because LiFePO4 safely uses about 80 percent of its capacity, you need only around 83Ah 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 840 Wh from your lead, and saves roughly 44 kg.
Fit: your Group 4D box is 527x222x250 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.