A 18-20Ah AGM type for a airhead or K-series beemer is ideal, just needs a spacer to keep it aligned within the stock battery box. Ample cranking amps and considerably lighter than the stock tractor battery. And not as fussy as LifePo lithiums.
Been using these for over 20 years in various Lavs, very happy. They generally last 5-6 seasons and because they are so common, often quite good deals to be found.
piet