Yes, definitely replace the forestay. There may not be a need to replace the CDI foil as the forestay runs through it. I installed a CDI FF2 on my Macgregor 26S. If there is enough length to your foil, set it up so the halyard top fitting spins on the shaft of the swage eye, instead of on the cable itself, which will cause stress and wear. I've seem pictures of installations with broken forestay strands where the halyard top fitting spins directly on the cable, because the foil was cut too short. This in my opinion is a big installation mistake, causing unnecessary wear on the forestay. I think this is important enough that if it were my boat and the foil was cut too short to not allow the halyard top fitting to spin on the swage eye, I would get a new foil. Otherwise you likely will have the same problem again. CDI provides really good customer service, if needed, you could give them a call and see if they would sell you just the foil.
Here's a picture of mine where you can see the swage eye shaft inside the halyard top fitting.