What year is your 20, and what year was the 22 that the jib came from?
If the 20 is a 1973-75 (masthead rig) and the 22 is a 1972-79 (also masthead rig) then if this is the small "working" jib, it should fit, not sure if a 22 Genoa (larger jib) would fit the 20. If the 20 is a 1976-78 (fractional rig), and the 22 is a 1980-83 (again, also fractional rig) tha jib should fit using the same cautions as for the older models. If the 22 is a 1980-83 actually, I would think the working jib would fit any year 20. I'd take some measurements to be sure, but..... try the sail, it should be obvious if it doesn't fit. Even it it does "fit", you may need to adjust or even move the jib fairleads on the 20 to use the 22 jib (then again.... maybe not?)