A few days ago my AC suddenly stopped working and by the time I got to my destination maybe couple minutes passed, when I stopped I could see smoke from under the bonnet so knew that something failed. Lifted the bonnet and the smoke was coming from the front of AC compressor. Looking at replacement options I found a used X308 compressor which was also a Denso 10PA17C unit so ordered that hoping to quickly swap it.
Removing the original compressor was an absolute PITA job, all 4 bolts were seized solid and were fighting me to the end. The last bolt of course snapped so had to remove the whole AC plate and drill it out. Compressor still seems ok but what happened was the clutch with its friction ring effectively worn down to nothing and it started metal to metal contact when engaging. This eventually lead to a lot of heat and the entire front plate rubber melted and separated itself. I guess these don't last forever, I believe the compressor is original and close to 30 years old now, so its not too bad.
The X308 compressor have a wider pulley(with more grooves) but that wouldn't be a problem, what was however was the mounting holes are in the wrong spot so the X308 Denso 10PA17C technically does not fit in x300 without taking it completely apart and swapping front cover(which would require a rebuild kit). I could have just moved the clutch to the old compressor but decided if I'm that far in, I may as well order some new parts.
These turn out to be unobtanium. The only part that is available is front plate. Eventually after many hours of researching I think I found a kit that will work with X300 Denso 10PA17C AC compressor - THERMOTEC KTT040013. The pulley for it is wider but that should not be an issue, belt will still sit in the front grooves only. Parts ordered and now waiting. Few pics below.
Of course bearing also got cooked but the main issue was clutch friction ring wear which is that black band in the pulley.

 

Right, need to correct my above post. The XJ8 pulley will not work with X300 compressor, neither will THERMOTEC KTT040013 or any other THERMOTEC pulley from what I found out. The XJ8 compressor pulley offset is wrong, same way as THERMOTEC KTT040013, its too far out for the belt to sit in the right spot. Technically it does work but the belt sits half way through on the edge of the pulley so only half the belt engages it so not ideal.
The XJ8 pulley offset is one grove out from compressor when the X300 pulley starts as close to compressor body as it gets.






I have searched and searched for anything that would actually fit correctly here and finally on 3rd attempt I found correct(well, kinda) pulley,
DENSO 10PA PULLEY WITH BEARING, 8 GROOVES, Outlet Diameter: 125mm, AP AIR PART NUMBER: 610-PB147.







So despite the pulley being much wider than original, it works perfectly, belt just sits in the 4 grooves closest to the compressor body. This is the only pulley/clutch that I have managed to find that fits X300 compressor.
The front plate is widely available though, part number AC AUTO AC-05DN13 so if your pulley/clutch is still in one piece and not butchered/burnt its possible to salvage it by just replacing the bearing and the front drive plate.
All works now and compressor still blows cold air.

 

Typical. Went on holidays and the AC belt tensioner bearing died, probably should have swapped it when replacing clutch but it was still good at the time. Anyways. Bit of digging on the bearing size, inner size is 17mm, idler diameter is anything around 75, found out idler from a Peugeot 206 is a perfect fit so swapped that quickly just now and all is well again.
Idler was ~£11GBP from local Halfords.




INA Belt Tensioner 326735290