This would seem to be the obvious solution, 2nd hand barrels and Air cylinders can be picked up from various places including that auction site not mentioned (for some reason) In the case of the Mk2 (as in the Mk1) the rifles serial no (or identity) is stamped on the trigger block and not on the actual barrel (as is the case of later Airsporter Mks) therefore, if the barrel and air cylinder (complete unit) were swapped in this way, only the person doing the change could ever be aware.
There are plenty of 'donor' rifles out there, I personally don't see a problem in 'maintaining' cars, motorcycles or old rifles in this way - if the donor has been broken up for parts anyway! (sorry if I have upset the 'purists' in any way)
The important thing, is that these fine old girls are allowed to exist/function and give their owners (and subsequent owners) pleasure for many more years to come.
Only saying like!
If it's the 'challenge' of actually changing the barrel - then just go for it, however, the rifle remains 'changed' or 'modified' and therefore not true to 'purists'.
ASM