Is that an old Brunner? I have not seen one of those old riveted tanks in years and even then it was on an antique! I don't think they have made riveted tanks since the early 20's so that thing is OLD for sure, regardless of how long it has been used or how well it runs don't trust it because that tank can get you or someone else killed!!! It certainly would be a good candidate for restoration for a museum piece but to use as a source for compressed air it would be extremely risky at best. I suppose if you want to clean it up, change the pump oil and REPLACE THE TANK it may make a useful compressor but if anything breaks it would be finished so don't invest much in it.
You might want to consider this- clean it up and paint it (riveted tank and all), replace any lines/fittings that may be in bad shape then run air line behind the old tank into a hidden new tank then take your air from the new tank leaving the old one intact but UNPRESSURIZED. You would have a real conversation piece for sure!