check to see if the little fan near the pilot is running. You should hear the burring of the fan when you try to light the furnace if you stand near. if you can't hear the fan running, pull off the cover check the fan for debris, rust, or the motor may have stopped working. Check the fan should be your next step.
Most likely the safety thermocouple has failed though it is possible the bulb or the pilot light has slipped out of position and the pilot flame is no longer heating the bulb.
Contact them at: Williams Corporate Office
250 W. Laurel Street
Colton, CA 92324 USA
T ' 909.825.0993
They will put you in touch with a local parts supplier, you need a conversion kit for this process, good because of the unit age.