Montesa Cota 248/349 exhaust front pipe

[COTA66]

Hoping someone can help me find a front exhaust pipe for my Cota 248.  it came as a box of bits and I have gradually rebuild it from the ground up.  The original 21M engine had a missing RH case so I rebuilt and fitted a 51M 349 engine.  Just dropped the rebuilt engine into the frame and discovered the 248 front pipe will not fit the 349 barrel.  The port stud spacing and port bore are all the same but on the 349 barrel the exhaust port is more recessed and at a different angle to the 248 barrel.  I am told that the correct part number for the exhaust I need is 5160038-02.  I have tried my usual suppliers (InMotion, Cycles Avent) but they can't supply either new or used.  Anyone got a suitable header I could buy?  Are there any workarounds?  I could modify a 248 pipe at a push but would prefer a factory part if I can get one.  Also need a side stand and engine to swing arm brace bars if anyone has any kicking about.  Thanks!