Fogstar do a dumb lithium 100a so a usable 95 a for £150 with Bluetooth for easy monitoring £175 , so they’re now comparable to a conventional lead acid but lighter and more usable amperage, but your 120A lead acid is only giving you 60 A ??? If your absolutely wedded to old tech a 160a lead carbon would be about the same price as 2 x 120 same sort of size though taller and would let you access 85% of its amperage still far more than your 120a pair combined