12 AH is the equivalent of 12000 mAH, the closest Lipo is 11.1 volt or 3S but remember a fully charged Li-po is 12.6 volts, this drops to approximately 11.1 under load, similar to a gel cell., you could go to a 14.4 4S with the risk of blowing electrics but the extra speed is a bonus
Be careful of the connection format of the batteries, you could have the two 12v batteries connected in series or parallel , if you put the new batteries in differently it could end in
Mad Mike has given a direct equivalent for a parallel set-up but only 1/2 the capacity, If the batteries are in parallel you would need 4 6000 mAH 11.1 all connected in parallel, if its a series set-up two 6000 6S 22.2Volt Li-po in parallel will give you eq.
A Lipo is smaller per mAH, so a larger capacity Lipo can be used and take up the same space
Nick