According to this site
http://www.radiomuseum.org/tubes/tube_7c5.html the 7C5 can be sub'd. No idea if that sub is still available though. Another site says a 7B5 will work as well. The rectifier or also the vibrator can be replaced with a modern solid state retrofit if that were needed.
As others have mentioned, valves are the least of the troubles when getting an old radio operational. The capacitors and anything with degraded insulation must be changed first. After that if a valve is needed, then if NOS & a sub is not available there is always used or possibly Russian.
One thing I have heard but not sure of is a slight difference in frequency standard between US and Europe. Someone with a direct American import can say if that is true and if something has to be adjusted.
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