A few days ago I finally received the Mk. V Gas Mask I had been looking for. Fortunately enough, I was able to find one without the filter attached on to the hose, making the whole process of removing the hose much easier (and less heart-breaking).
The unmodified Mask is displayed on the right while the modified variant can be observed below.
After slowly peeling away the white gauze-like tape and unwinding the metal wire holding it in place I was able to pry off the hose with no effort at all. This was the first step in the modification process and I am now currently waiting for a 'port plug' which will be used to seal off the now exposed inlet tube which once held the hose. A new inhale port will then be opened on the right hand side of the Mask to which a 60mm filter can then be attached.
I will post the drawn plans to said 'inhale port' in the following update.