Technically, you don't need the three-hole tang (no such thing as a two-hole) to add trapezes, but it's far stronger than the original one-hole tang. After 41 years of wear, I'd go with the new tang.
As for the fiberglass, there's no magic bullet that will bring the deck back to it's original color/gloss. Most "solutions" (like painting) will fill in the non-skid to some degree and/or make the deck a lot more slippery - a consideration since you're planning on trapezing.
Hobie Cats are durable, but a 41 year-old boat is . . . a 41 year-old boat. These boats were not made to last forever and they sail just as well "ugly" as they do "pretty". Save your money for stuff like new shrouds, blocks and running lines.
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