We had a good discussion of the Seeadler, and the Revell kit, here in the forum a few weeks ago. It's in the thread titled "Need advice on painting plastic sailing ship deck." I've just moved the thread to p. 1; it should appear just below this one. There are some links to sources about the Seeadler in it.
The Revell Seeadler and Gorch Fock kits both are modified reissues of the company's 1958 U.S.C.G.C. Eagle. As is discussed in that other thread, the kit doesn't really represent the Eagle; it's too short. I haven't been able to find any really good pictures of the Seeadler, but I question whether she really looked much like any of the three kits. The real Seeadler was a converted American merchant ship - not a German sail training ship. One of the links on the other thread contains a pretty clear photo showing the real Seeadler more-or-less in silhouette, and admittedly in that view she does look a lot like the Eagle (or the Gorch Fock). But I question whether the kit's hull lines and deck layout resemble those of the Seeadler more than vaguely. The individual modeler, of course, has to decide how important all this is.
I haven't seen any of the three Revell kits for many years, but as I recall there were two major differences between the Eagle and Seeadler kits. I think the Seeadler had open rowing boats in place of the Eagle's motor launches. And the biggest, most conspicuous difference involved the mizzen mast. The Eagle is a barque; the Seeadler was a full-rigged ship. Revell gave the Seeadler kit a completely different mizzen mast, with yards for square sails on it.
Some sources suggest that Revell added guns to the Seeadler kit. That may be true, but I don't think so. I think the original Eagle kit had a pair of saluting guns on the forecastle (which have long since been removed from the real ship), and Revell simply left those parts in place. I may be mistaken about that, though.
To convert the current Gorch Fock kit to something resembling the old Revell Seeadler would, in other words, require making a new mizzen mast, and its associated yards and rigging, from scratch. Quite a bit of work for a model that would only approximate the appearance of the original ship.
The folks who have Revell Seeadler kits seem, in general, to have found them on e-bay. That's about the best source I can suggest.
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