A 38Ah lead-acid battery like the Victron AGM Deep Cycle 12V 38Ah only delivers about 19Ah usable at the 50 percent depth of discharge that keeps lead healthy, roughly 228 Wh. Because LiFePO4 safely uses about 80 percent of its capacity, you need only around 23Ah of lithium to match it, at a fraction of the weight.
Best-value lithium replacement
ECO-WORTHY 12V 30Ah
Gives 307 Wh usable against about 228 Wh from your lead, and saves roughly 8 kg.
Fit: your Group U1 box is 197x132x186 mm, the lithium is 197x132x180 mm. Fits the original tray
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.