If the batt is definitely good I'd start with the easiest thing which is the motor itself and also check the gears that connect the motor to the engine for damage. You'd have to take off the starter clutch cover to get the motor out. You could check the bearing that's in that cover, check all the gears and then take out the two bolts that hold in the motor and check if it spins freely. If you want to take the motor apart and clean it and check it is just two more bolts. All handy stuff. If that doesn't work then maybe go on to the starter clutch. I haven't taken one out so I'd be interested in how you get on
