I remember those PZRWEST. Made by Marushin of Japan. Your's looks like an original WWII relic!! The ammo and a new mag might be hard to find. Keep checking Ebay. You could aways make a real MP40 mag fit. There are still plenty available. Check the Shotgun News magazine, Ohio Ordnance ,SARCO or International Military Antiques website.
The airsoft is actualy my 3rd MP40. I had an all metal non-firing replica from Japan but it didnt take the plug fire rounds. Several years later I bought another MP40 as a parts set minus the receiver and installed an 80% machined receiver. I had to cut out the magwell, cocking slot and sear hole. This took the receiver over 80% and made it illegal. I ended up selling it to a gun shop. I also had the MG42, MG34, Vickers, DPM, Sten Mk II, MkV, Sterling, (still have one converted to a Star Wars blaster) Brengun, Browning .30, Thompson 1928, PPsh41, PPs43, VZ25, the list goes on. I was big into collecting demilled weapons for a while. I even had a 3.5 bazooka! Demilled of course. Sadly I needed some quick cash a few years back and sold them all.