'Fraid of that. It seems the OEM motor design changed, to one that looks like your pix, and the kits went out of date with the old style. Must've been the gear case and contents changed a little when the motors changed from round to what I called "square." Maybe the better term is "flattened."
You're still way ahead of the game, finding the problem, sourcing the part, and installing it yourself.
Just be sure the power is disconnected while you work on this thing. It develops some pretty good force and moves faster than you'd think.
