[B&W Neil]

If the enlarger has been designed to have its cooling system running all the time I think you would be lucky to get away with doing otherwise. I suspect there will be a high bulb failure rate (are they expensive bulbs?) but worse of all the excessive heat build up could do damage to other components and wires within the head. Even if you do some experiments to establish a 'safe' bulb light time you can bet that one day you will forget and leave the bulb on for longer with disastrous effect :-(

Is having it serviced / repaired totally out of the question?

Neil.