Hi, thanks for the comments all.
I'll be taking it to our own workshop. We have a ceccato screw-type air compressor that should take the tank up to a handy 7.2 bar (104 psi). So that shouldn't be a problem. The tank is 50L at an estimate, but might be 60 at the top end.
I've had a look inside the tank, and to me it looks like a diaphragm type design, so i'll have to see about piercing a hole in the diaphragm since there is no way for me to remove such. Got a regulator on order so just need to get some fittings and reducing valves now.
By the way, one other question, for the cfm that an airbrush is likely to use, will 1/8" fittings be adequate? Or go for 1/4"?