You need to unbolt the drive end housing (nose) of the starter. It will have hex socket-head bolts countersunk in the face of the flange. Mark it so you can put it back in the same orientation vs the solenoid. Probably will have to also remove the solenoid so you can unhook the linkage and get the drive to slide off the end of the shaft. Should also be a lock on the shaft to remove after you get the nose off before the drive can slide off the shaft.
Do you have the correct drive end housing and starter drive?
Link to OM's conversion pages on ImageShak
http://img400.imageshack.us/gal.php?g=header4oq.jpg
Also see the stick thread at the top of this board for more information about charging, batteries, cables and glow plugs etc.
http://www.acmocbb.org/ACMOC_BB/showthread.php?t=844