Wouldn't the Bren Gun from the Resistance or B.E.F. set stand in for a Madsen at this scale?
No. Well, maybe, but only in the sense that a Sterling SMG can stand in as a laser blaster.
The barrel, receiver, magazine, the stock etc all look quite different. There would be so little left from any rebuilding that you would be better off scratchbuilding it from plastic stock and rod, using a rifle holding arm, as the Madsen did not featire a pistol grip. As it happens I have some 28mm metal Madsens floating aroud in my spares box.
Late war and RCW era Russians could use the Lewis from the British pack. Ultimately of course what folk do with their toys and what level of fidelity they feel towards accuracy is their affair. Your toys, your choice.