It’s not the voltage, it’s the cranking amps that count when starting so may try going for a battery from diesel, these usually having higher values, Ijust went to the local motorfactors and asked for a battery to fit the standard fixing clamp for my 1500 precrossflow, they offered two, one was a bit tall with 350amps cranking, the other was just right but only 270. I bought the latter but should have bought the former, it starts it but only just.
They did say they’ll swap it for me.
Nick