The Cal 20 isn't a very heavy boat, you could use beer and several big friends. Of course this means a stripped down boat, no engine, tanks, rig, etc. In short anything that can be removed or is likely to tumble about when rolled onto her deck, should be.
A heavy duty come-a-long or a light chain fall can do the trick. Rig up a sling (two towing straps) around the boat, find a healthy tree with appropriately large branch and hoist her up a few inches. Once clear of the trailer you can swing her around easily and roll the trailer out from under the boat. It will take some fussing to find a good balance point on the hull with the straps, but trial and error will find it pretty quickly.
Let the boat down on some padding, then adjust the straps so she will be lifted sideways, then hoist again. Place her back on the ground (padded) and continue working her over, little by little, until you reach the point she'll want to balance on her side. When this happens a little push in the direction you want her to flop is all you need. Let her down slowly and adjust the straps to lift her up square with the cradle or strong back or what ever you'll be putting her on, for the bottom work. Getting the boat back on the trailer is basically the reverse of above. It's sounds difficult, but it's not that bad a thing. Work safely and slowly. Use towing straps rated for well over the weight of the boat, ditto the hoisting gear, you don't want it to fall, trust me, I've dropped more then one boat (look, after 30 plus years of repairs, you're bound to drop a few)
A lot of the restorers around here (myself included) will flip your boat and place it back on the trailer for a small fee. It's likely you have the same available in your area. I use multiple chain hoists to get it done quickly (lowering one side as I raise the other), but you can do it with one if you're not afraid of a little effort and some planning.
Good Luck . . .