I've only just started to modify our Catalina 320 ... in the process of replacing engine parts, I've had to bump out the engine cover at least 3/4" (very minor mod). I modified our Starwind 27 so much that many might not even recognize it (hardly anybody knows this Jim Taylor design anyway).
The black paint on the spars was deteriorated and I preferred white, so I painted the spars white. Same for the binnacle, which was black, and I painted it burgundy. I changed out all of the black plastic-framed lights and hatch on the boat with custom aluminum-framed lights and hatch. I increased the size of the ventilated lights in the head and vanity and added an additional pair of ventilated lights in the v-berth. I changed the 1 cylinder engine to a 2 cylinder engine and had to modify the engine compartment for 2" additional length. The entire exhaust system was upgraded and a new aluminum fuel tank was installed.
I removed the galley sink and put in one that was slightly larger. I added pressure water and a water heater. I removed the aluminum water tank and installed a stainless steel tank (increased the size slightly). I upgraded everything in the electrical panel and added electrical fixtures. I built new cabinets for the upgrades. I replaced everything for the toilet and plumbing (except the holding tank). I added equipment and rigging for a sym spinnaker and added roller furling. I improved the traveler track and system. I replaced all rigging at least once and added backstay adjustment blocks. I only replaced sails once in the beginning. Modern chartplotter and electronic navigation equipment were added to the original instruments that could be restored.
I restored the bottom entirely, starting from scratch with new barrier paint. I never did get to repairing the soft spots on the side decks. I did grow a little weary in the end from polishing and waxing the topsides every year.
After 15 years of ownership and about $40 K in improvements, I finally sold the boat for about $2,500 more than my original purchase price ...

The boat was still recognizable as a Starwind 27, I think, only it was the most thoroughly up-graded Starwind there ever was.