So the enlarger (I'm not familiar with that model at all) is depending on the outer envelope of the bulb to act as a diffuser? All the ones I've seen have either a mixing/diffusion box or the condensers are designed to be focused at infinity wrt the bulb, i.e. they convert a point-source into parallel rays.

Anyway, you should be able to find a ~100mm white glass sphere at a lighting shop and then place a much smaller bulb inside it if that is the required effect. You may have to forcibly ventilate the air between sphere and bulb though to prevent overheating of the bulb.