switch can be transferred
I just did this on my 87 SDL about 5 months ago. The rebuilt compressor had a bad switch on the back - speed indicator sensor I have been told. If I remember, it has a rubber cover, then is held in place with a cir-clip. The kind of clip that goes 80% or so around a circumference of the hole and has holes for special pliers on the ends. You pinch the ends together and it comes out of the slot it rides in. The switch then comes right out. My friend and I were able to transfer the one from my siezed compressor to the rebuild very easily. We held the compressor pulley down / switch up and did not loose any oil.
If the compressor bolts up ok I would say not having some of the flanges / bosses should be fine.
My pulley was slightly larger than the original. I bought a slightly longer belt.
Fingers crossed, it is still working just fine.
One later thought... The rebuilt compressor had the rubber switch cover in place but no wires showing - this may appear that the switch is missing. In fact it appears that the sensor provided in the rebuilt had been damaged and the wires were clipped off to prevent its use. <end edit>