There are probably similar-shaped resistors available - look at the Mouser, DigiKey, Allied, etc. catalogs online. Something the same size and shape will heat up similarly. It would have the same rating if the maximum internal temperature were the same, but of course modern resistors are probably able to run at higher temperatures. Download the spec sheet for the similar modern resistor; there will probably be a de-rating graph for lower temperatures. For what it's worth, I run modern resistors at no more than 1/3 of their maximum rating, just to make sure they don't get extremely hot - the resistor may handle 250 degrees C, but the capacitor next to it can only take 85 degrees!
That still leaves you guessing about the materials used and the maximum temperature of the old-stock resistor. You might get more help from the vintage audio board on the Audio Asylum.
