Hi,
I have just been dismantling my wiper motor assembly as I need a new brush pack.
If I understand correctly you have the worm gear on the new motor, but to replace the old motor with the new you need to remove the sprocket which is rotated by the worm gear.
It seems to be a very thin circlip/E clip which just clips on. How I removed it was to use two stumpy flat bladed screw drives and push the clip ends towards you with your thumbs on the opposite side of the casting, I eventually managed to remove it, but yes it is a very thin clip. Also be carefull not to loose it!
Hope this helps
Steve