The stages have a reverse DIN type fitting and this is where you problem is gonna be.
The rest of the 1st stage is standard (02 cleaned of course) but the fitting is not standard. Unfortunately it's not the same as the Dolphin, as that would make life easier.
Blandford Sub Aqua sold me my unit some years ago and they may still have some bits.
Have you got the cylinders? If not then you can start fresh really. Can your Azi instructor not help you at all?