The 21's have a TON of stuff in common with the 18's. Many, many of the parts are interchangeable, I found out to my pleasant surprise. Couldn't find the boom length real easily. If I were a manufacturer and could make that interchangeability work (read lower costs) I'd sure do it.
I'll measure my 21 SE boom length, check that against an 18, and I'm betting they're the same extrusion, but the end caps would likely be very different. At the least, you could forgo the furling main sail and raise and lower like everyone else...but then, you'd be like...everyone else... It'd get you sailing, though. I believe the gudgeons, castings, etc. are exactly like the '18. The rudders even work, apparently but are a bit shorter. You may want to check our Hobie.com on their forums as well for more rebuild info.
While a heavy boat, I sure love mine! Just need more time on the water...