The gas piston being under the barrel wouldn't work very well, it complicates things. That's why the StG 44 and AK-47 both had them on top and even the M16 eventually changed the gas tube to feed from the top. It can be done though so you shouldn't change your whole gun around for it. The entire gun looks more like the StG 44 than an AK-47 which isn't a bad thing.
You need to specify what you mean by a "drum magazine" since there are several things that are roundish and feed ammunition into a gun. The thing the Bizon uses is a helicald rum magazine. The MG 34 used a simple saddle drum that just held the coiled belt of ammunition and then a lot of modern firearms can accept Beta-C magazines that feed like normal ones. - Colt