The aircraft designer Aarne Lakomaa turned the obsolete "M-S" into a first rate fighter, the "Mörkö-Morane" ("Mörkö" is the Finnish for "Bogeyman" or "Bugbear"). It is sometimes referred to as the "LaGG-Morane". Powered by captured Klimov M-105P engines (a licensed version of the HS 12Y) of with a fully adjustable propeller, the airframe required some local strengthening and also gained a new and more aerodynamic engine cowling. These changes boosted the speed to . Other changes included a new oil cooler taken from the Bf 109, the use of four belt-fed guns like the M.S.410, and the excellent MG 151/20 cannon in the engine mounting. However, supplies of the MG 151 were limited, and several received captured Berezin UBS guns instead.