Determine power requirements and limitations for an old shortwave radio
Since you know the voltage, you can safely power the radio.  Use a 9 V bench supply or something, and measure the current.  If this radio has its own speakers or is intended to drive speakers directly, then run it at maximum volume to get close to the likely worst case current.

Then get a 9 V regulated wall wart or power supply that can supply a bit more and round up to the next higher available current.  For example, if you measure 421 mA worst case, get at least a 750 mA or 1 A power supply.  The difference from 500 mA to 1 A won't cost much extra.