On my 1300 the water enntrance was simply riveted in on the four corners.
I would imagine that if you see something additional schmeared? on from the inside that it would be caulk of some kind.
I would drill out the rivets from the outside,being careful to use a sharp bit and light
pressure so you don't ream the holes out bigger when you get thru the rivet.
Then if it will not move try a putty knife or 5 in 1 tool to gently pry the flange from the glass.
If there is caulk on the outside you can lightly score with a razor knife angled under the flange but try not to cut thru the gelcoat.
Its funny but I just replaced the water entry on my new Fiberstream
yesterday and I could not find one the same size so I had to enlarge the hole a bit.
The Roto-Zip is a great and tricky tool!