There are all sorts of ways to do this. I have a Kavan starter that I rebuilt/repaired that originally had 10 sub-C nicads in it. The batteries were bad, so I looked for and found a deal on two 6 cell R/C car packs and then collected them in series. This gave me 14.4 volts ( the extra volts do not bother anything) and I think they were 3800 mah cells. I just had to work out a way to mount them and some black duct tape helped with that. These all used the common molex or what some call Tamiya connectors. Hobbico had an add on battery box and battery to make their starter cordless and there are a lot of them t there in swap meets. I don't know if they are still available from Tower/Horizon Hobbies or not. Some one here on the list mentioned a system for sale that uses the battery from a Makita cordless power drill, ( I think it was Makita) but the adapter alone is 50 bucks. You can find surplus 12 volt gel cell batteries that are relatively small and 5 amp capacity and can be held to the bottom of the starter with zip ties. four cell Lipo batteries will get you in the neighborhood also. Look at what you might have on hand for chargers and such and maybe start theirs since you already have that.
Type at you later,
Dan McEntee