To my knowledge the only hospital ship kit produced is the old Revell U.S.S. Haven, which came out in the late 1950s. (I built one when I was in grade school; I guess that dates me.) It was re-released as the S.S. Hope in the early '60s, and again as the Haven, in a reproduction of the original box, in the Revell/Monogram "Special Products" series just a few years ago.
I'd have to do a little digging, but I don't think this class was identical to the Mercy. The Haven was in fact an Army hospital ship; I believe she was built during World War II. The Revell kit represents her as she appeared when the kit was released - with a helicopter deck on the stern.
Sorry I can't be of more help.
Youth, talent, hard work, and enthusiasm are no match for old age and treachery.