Its nowhere near so simple. There are just too many unknowns for any book of tables to help.

The enamel on the wire has a temperature rating for 10000 operating hours. The I^{2}R losses, that temperature rating , and the thermal resistance to ambient determine the maximum current the wire is rated for.

All the I^{2}R (copper) losses in the wire and the iron losses in the core must be dissipated from the surface of the toroid. This is complicated by the potted base, which is likely to be poorer at conducting heat away than convection from the winding surface in free air. A further issue is the unknown wiper current rating.

The best way forward would be to determine the max temperature rating of similar Variacs of comparable dimensions from the same manufacturer. Initially measure the cold condition DC resistance of the winding and cautiously apply low voltage DC to the whole winding to heat it with a known power, while monitoring the surface temperature rise with an IR thermometer or a thermocouple or other sensor applied to the surface with a dab of thermal grease and shielded from the ambient with a small block of polystyrene foam. Note the temperature when it reaches a steady state.

Next power its winding normally from the line with no load on the wiper and either measure the input power if you have a sufficiently sensitive AC wattmeter or again determine the steady state temperature, That power or temperature rise above ambient is the iron losses + that part of the copper losses due to the magnetising current.

Given the ambient temperature, the max winding temperature, the temperature rise per watt, and the unloaded losses (either as power or temperature rise), you can calculate the maximum copper losses it can tolerate. That would give you a maximum current for the whole winding from I^{2}R and the winding resistance at maximum temperature (calculated from the cold resistance using published resistance vs temperature tables or formula for copper) .

Due to auto-transformer action, the current in the winding is locally at a maximum when the wiper is very near (but not at) either the N end or the tap where L is applied, and for a conservative rating you need to keep that current under the maximum current calculated for the whole winding. The odds of the wiper rating being less than the winding rating are extremely small, and as long as the damage isn't near the ends of the winding or any tap, hot-spotting due to the nicks in the wire should be insignificant as the current through that section of wire will always be significantly less than the calculated limit.

If you've got access to a calibrated thermal camera, you may be able to push the ratings higher based on observing the wiper temperature rise and checking for hotspots with a known load and the wiper at half the input voltage, then re-checking at 90% of full voltage to get an estimate of how well local heating in the winding is dissipated over the whole surface by conduction through the core, then recaculating the maximum load current vs wiper position, so that no part of the winding or the wiper exceeds the temperature limit.