Starter Question newbie help

[townie]

hi all im a newbie
just bought a used sunl 2007 with a 150cc and the battery was weak so i decided to hook up the battery booster box and to my suprise i sheared the woodruff key for the starter clutch stupid me but now i need to find a new one does anyone know the size of it.
and also how big of a battery can i use on the engine if i modify the battery holder thank