Modern cordage in 6mm is likely to be stronger than the wire you have now, depended on what is selected. Your local rigging
shop can advise you on whats best. Count the strands in the wire halyard, measure it's dia and go along to the rigging
shop and tell them what you are doing. They will be able to tell you what is best from the products that they sell. Dont forget that the sheave in the masthead (an possibly the foot - is it an internal halyard?) is likely to be a a sheave designed for wire - they can be hard on ropes, if in doubt, take them out and with you to the rigging shop...