If your mast is Z spar US Spars, contact them, They should have all of the dimensions for the original standing rigging.
Running rigging specs are in the owners manual.
I would recommend taking the mast down and replacing everything while on the ground. Much safer that way. Check everything attached to the mast while its down.
Your jib furler has to come down to replace the forestay, and because of the connecters between the furler sections, depending on brand you will not be able to remove the forestay without cutting off the stem fitting, and when you pull it out, tie a messenger line to the end so that you can thread the new forestay through before installing the new fitting.