I have just completed fixing (I hope) some old portlights. 3 leaks were caused by faulty gaskets; for one I had to rebed the frame. I did use a putty knife, but a very thin one with rounded edges. Worked a treat. It was also useful to have a friend helping, gently pushing from the inside while I pried from the outside. Mine was bedded with caulk and 2-sided rubber tape(!!!).
For you, the key question is what bedding compound was originally used. Caulking is easy to deal with (but a pain to remove prior to refitting). If it was 5200, you may end up replacing the portlight; but I have been told a Dremel with a variety of cutting tools may release it.
4800 is a good option. I used butyl tape, which I am finding easier overall to use than caulking, and am told if is a great performer over the long term. A friend who rebedded his portlights opted for Silkaflex, as he was concerned that the large void around the porthole would be not be as usefully filled with butyl tape.
Installing New Found Metals Stainless Portlights Photo Gallery by Compass Marine at pbase.com
is not totally relevant, but worth looking at!