@Red alert
Another "Left of centre" thought! If the correct replacement arm is not likely to become available, then how about cutting the current arm into 2 sections and inserting some form of "extension"?
If you can find an approx 120-150mm length of steel (or aluminium) channel that will fit fairly snugly over the arm then you could bolt that to the dish-side section and the LNB-side section to provide the extension.
You might even get away with steel straps bolted on both vertical sides of the 2 sections - and if you used perforated steel straps you might be able to incorporate an adjustable extension so as to be able to "fine-tune" the arm length (perforated steel car exhaust straps come to mind here)
Words of warning : if you do this, you would need to cut the arm as near as possible to the LNB end to minimise the bending moment on the extension piece, and drill the holes for the securing bolts before you cut the arm as it will be far more rigid before than after cutting. .