need coolant expansion reservoir cap

haven't bought the new (to me) '73 v20 yet, still have to check out the outdrive saturday (boat is getting summerized), but the one problem bugging me with the motor (besides the fact that it's 25 years old) is that the cap is missing on the coolant reservoir. tried all over the net, nobody has one. anybody have an extra to sell, or a rec as to the type to use from another engine? 1979 470 merc. thanks.