The original furling set ups on the early SC20s were from Ronstan, less pricey than the Harken set up you have pictured. I recently bought the drum for less than $60 from Rigging Only, and I believe the upper swivel was about the same money. Rick can probably get them as well at the OnLine Store. They worked fine on that boat, and I have the pictured Harken drum and swivel on Flight Risk's screecher. It's more than you need for a small cat jib, although it will work fine. Your forestay will need shortening, which might make your existing jib need modification. I'd recommend holding off till you're willing to have a new jib made specifically for furling.

Dave