Easy way to estimate the power handling capability of an unknown transformer
If you have an oscilloscope, an ammeter, a variac, and several loads, you can crank up the voltage on the primary side and measure the waveform of the secondary under increasing loads.
When the waveform starts to distort, the core is saturating.  Watching the current on the primary to make sure you don't exceed the capability of the input wires is also a good idea.
Once you have a working input voltage, run the transformer at about 80% of the saturation load for awhile to see if it gets too hot.  (Do this on a non-flammable surface, with a  fire extinguisher nearby.)