blow out

next step was to apply heat to the damaged areas with the aid of 2 butane/propane blow torches to soften them metal for the internal air pressure to push out the dented areas.

I got the first pipe fairly well sorted with a few of the deeper scores still visible, The second pipe was more of a challenge as the damage was greater . . . and I ran out of gas!

pumped

Next step was to plug both ends of each pipe after removing the baffles pressurized with air from a foot pump.

I used 5mm steel plate and an old bicycle tube as a seal with various brackets and clamps. I was able to get the pressure up to 80psi.

compressed expansions

A major concern for me was the whole topic of the expansion chambers, firstly sourcing a set and secondly fitting them with the non native frame present.
I managed to source a set from a forum member on RDnuts . they are a pair of early Microns under the name "Codnor Micron" with short can silencers. They have been  involved in a meeting with a foreign object at the diffuser point and need to be repaired.