Was doing some research yesterday and some people were saying that the bottles were CO2 bottles originally, hence why they have 'FAC' on them (CO2 guns were FAC only pre-1997). So perhaps some kind of CO2 bottle fitting may work.

As above though maybe this is a non-starter; biggest issue I think being the magazine holder. While I would be loath to loan mine out, if you did want to persevere with the project I'd be happy to take mine off the gun and take any and all measurements you may need. It's not an incredibly complex part from what I recall.

The light at the end of the tunnel is that clearly spares are extrememly thin on the ground, so you may be able to recoup your losses breaking the rifle for spares. Sacrilege to some, but I'm sure there are Skan collectors who would have use for the parts...