If you don't care how much work it takes, but don't have access to an iron foundry, you could cut out the thin area of the drum all the way across it longitudinally, weld in a chunk of cast iron, then turn and bore it until it's drum shaped. You'd probably need to do some more welding at that point. Getting some other drum the same diameter and cutting off the cylindrical section then welding it to the closed end of the old drum would be a lot easier. With the clutch you could weld in a block of cast iron then turn it to the right shape, but it might be easier to just turn up a new one out of bar.
I suggest you measure up the carburetor and think about three alternatives: get a replacement carburetor; tap the stripped one out a size and turn up a replacement part to screw into it; or helicoil it.
