MS.406 series were tube frame fuselages with metal and Plymax (aluminum skinned plywood) skinning. The MS.450 (which begant the MS.540/D-3802/D-3803) had a monocoque fueselage with Plymax covered wings. The MS.540/D-3802/D-3803 were all metal and much modernized. And indeed, engine issues (trying to milk the last out of the Hispano-Suiza 12Y like the Soviets did with the Klimov M105 and later engines) and simple cost/benefit made it appealing to buy P-51s and later DH Vampires when they could get their hands on them.
The Mustangs and Vampires replaced a motley assortment of Doriner MS.406 developments and early and even later Me-109s.