Hi David,
MK did not kit an Arrow 20. There are some other 'gaps' in their kit production:
Skymaster 60, 20 no 40 size
Joker 60, 25 no 40 size

Conversely, MK catalogs list 20, 40, 60 and 4C sizes for both the Magic and Aurora; though I'm not sure what differentiates the 60 size from the 4C size (dimensionally they are the same).

MK also produced FRP (fiberglass reinforced plastic) fuselages/kits for the Magic and possibly others (which were never imported here in the USA).

-Will