www.harriscyclery.com has a few offerings under "freewheels" that would probably
fit. Big question will be does the rear hub have "English" or "Italian" threads. Maybe
your father knows. Merckx bikes have been made by a wide variety of manufacturers, large and small over the years. The Harris freewheels are probably
all English threaded. www.sheldonbrown.com should have a discussion of threadings
for various parts of the bike that may give you a clue as to which you might have.
IIRC the Campy BB threaded parts should be stamped with their thread size, if this
is the Italian BB thread it would suggest the wheel is also, and ditto for English threading. Steve