I had the same experience...and countless others too probably.
There is also a small seal that sits inside the drive hole - looking from the outside.
This will be hard to budge after years of heat exposure etc.
I put plenty of plusgas in from outside to create a small reservoir by tilting the cover and left it to soak overnight - any similar un-seizing juice will do.
Then used an old screwdriver as a wedge between the cover and the geared end of the shaft.
You need to get enough force on the end of the shaft to push the seal out....
Be careful not to damage the hardened teeth though. No hammering !
Once a gap opens up you can get small strips of aluminium or copper in the gap to protect the cast Al cover and the gear itself. It will come out !