12V lead acid battery instead of 11.1V LiPo?
The output voltage from a standard Li-Ion or Lipo battery pack with (3) cells in series (3S) ranges from about 12.6 Vdc (freshly charged) down to about 9.0 Vdc (end of life cutoff-voltage).

A standard 12V Lead-Acid battery ranges from about 14.5 Vdc (freshly charged) down to about 11.0 Vdc (end of life cutoff-voltage.

Best to check the datasheet for the device(s) that you are powering.  However, my past experience says that you can safely substitute a 12V Lead-Acid in place of a 3S Li-Ion or Lipo battery pack.

[Edit]
One thing to consider that Li-Ion or Lipo batteries have FAR greater energy storage as compared to a Lead-Acid battery of similar size or weight.  Lead-Acid batteries are significantly less expensive than Lithium technology.  That is one of the few advantages that Lead-Acid batteries have compared to Lithium-type batteries.