You can probably come up with a boom from these guys. Seal's Spars and Rigging
If I remember correctly, the boom on the Cal 20 is the same length and design as the 21.
I probably wouldn't be sailing that boat until you get the framing back in it; you're missing the entire structure between the chain plates! When I haul my boat I'll take some pictures so you can see what you're missing in your's.
The good news is that everything is more than accessible now!
Here are some photos I took today of the winch...I didn't have time to pull the keel stopper but will take a photo of it so you can relate to the drawing you have of it.
I can understand your desire to move the sheet but you'd have to go to mid-boom sheeting so as to clear the tiller and I think you'd find that the traveler was then right in your way. I suspect that the pushpit that has been installed on your boat might interfere with the main sheet. The boat did not come with it or the lifelines. If you want to keep the lifelines, I'd secure them to the gunwale and get rid of the pushpit as I think it's just going to be in your way all the time.
Here is the keel slot. I had a slight leak between the cockpit deck and the keel slot. It leaked only when the boat was sitting on the trailer and not when it was in the water but it drove me nuts. I added the stainless steel flat stock that you see in the picture to reinforce it above and below after I separated the deck from the keel trunk and resealed it.
Here's a closer photo of it. Note that the winch you see is actually sitting athwartships across the cockpit seats and you're seeing it's reflection in the water in the keel well. You can see the aft end of the keel stopper secured inside the trunk by a wing nut to the threaded bolt through the aft end of the well. The portion you see is about a foot long and the hinge is on the bottom of it. The keel stopper is tapered at it's forward end and extends all the way forward to the aft end of the keel when the keel is in the lowered position. If you can get your boat up on boat stands, you can lower the keel and dry fit the stopper after you fabricate it.
Here's the winch and it's board sitting nearly in position for lifting the keel. You'll note the grab and go hook on it. The winch board spans the seats when raising and lowering and if the keel swings freely even your wife should be able to raise and lower it. Of course, the big concern is keeping a good grip on the winch handle and not letting it fly!
Here it is upside down and you can see the locating blocks and the hook. The light line attached to the hook is the release line for tripping it when the keel is lowered.
Here's the footing that covers the keel well once the keel is lowered and the keel stopper installed.
This is a view looking forward in the cockpit; you can see some of the interior that you're missing. I'd have moved things around and taken a better photo if I'd known that so much of your interior had been removed.
I'm very serious about not sailing that boat until you do quite a bit of structural work on it. You're missing two or three frames and the keelson forward of the keel trunk. The aft end looks marginally better but it appears that the aft frame has been cut out when the settees were removed. The boat is seriously structurally compromised as it exists now. You've quite a bit of marine plywood and fiberglass cloth work to do to restore her to a sound condition. It's all do-able of course if you've the time, patience, and a modest sum of money.