There is a train of thought here that says it's a 20HP, therefore willprobably have a pull start for emergency use and I bet it is "self sustaining" (i.e doesn't need the battery to stay running once started).
My rib came with a super duper deep cycle heavy duty etc etc battery that I discovered would cost about £120 to replace like for like. One batt failure halfway through helping out at an event and a trip to the local Halfords for their £40 cheap N cheerful battery to keep it all going & I've not had a problem with it since.
A 20Hp won't pull anything like the cranking currents that some of the bigger three figure horsepower machines draw when the key is turned.
I suspect this will cause all manner of "you can't do that" kind of replies, but for a smaller engine in a sub 4m rib, there's almost an argunment for using a motorbike battery & keeping the weight down.