The MP5 stock will fit on the 53/32 receiver, but it has to have some sort of buffer in it, especially for the 7.62x39 round.
But that is the least of your problems with the SW32's. The barrels in the SW32 rifles are desinged wrong, the chamber face is milled concave, which can sometimes make the rear of the case rupture and explode. Also the recoil will tear the gun apart. I suggest you have someone rework the entire gun.