It's the same part as a Hobie 18 - #50404101. I think it's the same one on a H21SE as well. You may have to get it used, but it's just an 18" section of aluminum extrusion.
Process is - drill out the rivets holding the aluminum mast head - you just want to drill out the head of the rivet and not drill into nor widen the hole into the comptip nor the aluminum of the head. If you do, the new rivets won't fit quite right. I drill with a slightly bigger, SHARP drill bit (dull will make the rivet spin) using a light touch. Then when the head of the rivet spins off, I use a small drift punch to punch the rest of the rivet inside the mast. Should leave a perfectly clean hole. The mast head should then come out - it may be sealed in with sealant of something, meaning it MIGHT take a little effort to free it. You slide the head off and the luff tracks just slide on out that direction. The mast head is holding the plastic and aluminum bolt rope sections in. Measure on the plastic how long your new luff section is, cut that off of the plastic luff track, smooth edges so sail doesn't get caught and slide the plastic one on first, then the 2 aluminum ones on next. Put mast Head back on and rivet it in place.
Really that simple - took me about 2 hours doing it very carefully and slowly. I re-sealed my mast and was careful to use closed head rivets. Worked fantastic and I never had the bolt rope pull out again. I got mine through Murrays, back a couple of years ago - not sure if they have them anymore, nor if Hobie even has stock or produces them. While expensive ($84 new) for a chunk of aluminum, they are worth the price with reduced aggravation, etc. If you can't find a new one - scavenge an old one from an old mast somewhere. an H17 has a little longer one, but I'm guessing it'll fit also - just cut more plastic out.
I'd call Murray's or your local Hobie dealer and see if they can get it -
https://www.murrays.com/product/50-50404101/