To be honest It'd be easier to just get a larger Ah SLA battery and a decent charger. Especially if you aren't looking for more speed

Using these types of batteries just adds degrees of complexity that In my opinion leave more points for failure.

Even upping the speed by increasing voltage is far easier by adding a 6v battery in series to achieve 18v.

To answer the question you need a converter than can handle 30 amp minimum, but up to 40a as depending on weight and terrain the motors can draw over 30a.
https://www.amazon.com/uxcell-Converter-Regulator-Transformer-Waterproof/dp/B01LYK6G2Y/ref=sr_1_5?crid=1M76GD94KMJYW&keywords=dc+buck+converter+12v+30a&qid=1652728750&sprefix=dc+buck+converter+12v30a%2Caps%2C95&sr=8-5