By the time the Q came to be, Alfa had switched over to R-134a. New compressors are readily available, however, SAVE THE ORIGINAL PULLEY. You'd need to reuse that. If opening the system to replace the compressor or the front seal, I'd highly recommend that the condenser be replaced with a modern parallel flow type. It's something like 30% more efficient than the OEM one.
This should be the part you'd need, but should check with the service guy. It requires making a couple "feet" for it and trimming a few pieces of plasticwork, but works much more efficiently than the OEM condenser. Full write up for fitting it is in the 12v R-134a Conversion Guide.
I'm not certain this applies to the Q, but I'm betting it does . . .
VERY IMPORTANT NOTE:
It's likely that if your service guy has to remove the compressor, he's going to be extremely miffed (not the language that he'll use BTW) when he can't get the long top bolt out because it hits the body work before it comes out enough to remove. The reason for this is the engine was installed in the car with the compressor attached and the engineers did not see the service problem that would arise later. They could have specified installing the bolt the OTHER way and it would be no problem, but they didn't. Now it's the mechanics problem. See page 5 of the Conversion Guide, for a picture of the issue.
You DON'T want the mechanic to get all the way down to removing the compressor only to find out about this issue and having the car in pieces until a new bolt can be obtained. Get it first, then start work, assuming he has to remove the compressor.
The solutions are:
1. Jack the engine up enough to clear the inner fenderwell. Time consuming and expensive. Not recommended.
2. Beat the fenderwell down to the point the bolt can be removed. Not recommended either. We don't beat our cars with nine pound hammers.
3. Cut the bolt and install a new one. Best solution, BUT, make sure you have a replacement on hand. That metric bolt is available (10mm diameter x 1.25 pitch x 170-175mm long (1.5 pitch will work also, but you'll need the correct nut too), but probably only from a specialty fastener supplier. An inch spec bolt is commonly available 3/8" and could be used in a pinch, but is slightly small in diameter. As long as the compressor is pushed all the way against the engine and then tightened, it should work ok until a correct bolt can be sourced.
4. Cut the bolt head off, and pull it through from the driver's side. Then center weld a nut onto the cut end. This is pretty backwoods redneck ugly, but would probably work.