I was going to make a new housing for the LEDs but now I think I will modify the original head. My LED PCB is from an LED stage light and is about 7" diameter with red, green and blue LEDs fitted. It should fit in there with no modification. Just four pillars to mount it on. I will probably make an intermediate plate which the PCB fits onto with four bolts which fit the holes previously used to mount the transformer.
That way, if I ever find a replacement tube, I should be able to swap it back again.