Your center plate may still be stuck to the right case half. Make sure it's loose from that side before you try too hard to split the cases. The center plate has to stay with the drive side (side the clutch goes on) until the clip is removed from the end of the input shaft.
HMike's right about the Onan- somewhere I've seen a pic of one mounted in the back of an Attex.
Stuck in the seventies- not in the swamp.
(6) Attex, a Hustler, a Super Swamp Fox, (2) Tricarts, (3) Tri-sports, a Sno-co trike, 3 Dunecycles, and a Starcraft! ...so far