You may need to also replace the block (pulley) on the mast as well, but other than that there should be no reason that you can't replace the wire/rope halyard with an all-rope halyard.
Boats (especially racing classes) used wire for part (usually one half) of the halyard length to take advantage of the reduced stretch of stainless cable versus the 3-strand polyester rope of the time. Modern rope, even the least expensive double-braid polyester, has much less stretch than older types. Racers may use high-tech synthetic rope to gain even less stretch, but the old wire/rope halyards are being replaced by all-rope on most boats.
I use 1/4" double-braid polyester (Dacron) for both my Main halyard and jib halyard on my Day Sailer. Main halyard is 50' and jib halyard is 36' You may wish to add a couple of feet to each of those lengths to allow for splicing the shackle on and since the DS I ran the halyards to cleats on hte cuddy top, mine cleat to the mast.