If I remember, my MC2100 had an intake that was the same as a 4-bbl -- that is, about 5-1/8" in diaameter. So just about any air filter that would fit a 4-bbl will work. Measure from the center mounting stud on the carb to the nearest obstruction in order to figure out the diameter needed.
BTW, I'd recommend a regular dry air filter over a K&N. I removed the K&N from my 360 some time ago because it let in too much crap.
1977 CJ7, AMC360 V8 w/ headers, DUI HEI, Edelbrock intake, Holley 4150 carb, , TF999, D300, D30 front and AMC20 rear with 4.56 gears and lockers both ends, 4" susp lift, 2.5" body lift, 35" BFGs, ARG steering, 8000-lb winch, etc.