I had the same issue with my C&C 36. I went from a wire/rope combo to a all rope rig. The new ropes are strong enough to replace the older system.
The bad news was that my messenger line came off half way throught the process of pulling the new halyard through the mast. If I do this again, I''ll hire a rigger to do the job. It''s cheaper than nightmare you get if you do it wrong.