Heres something to think about. If the original designer of this unit had been able to put a led in it, do you think he would have?
As to soldering a bulb in, thats just a really bad idea, because you will have to replace it again at some time. Those fittings are still available, just not quite the same, used on Fender amps and the like. If you must have a bulb, replace the fitting with one you can screw an available bulb into. Dont forget, whatever you do, you cannot buy the original bulb. So you are modifying the unit.
Also dont forget, the EEC wonks are forcing manufacturers to stop making incandescent lamps, so they will get harder to find.