The same compressor is used on ac couple of mid 90s USA build Honda's. I found rebuilt compressors in the USA, with the right pulley, for 100-200 dollars.
My car had a new compressor in 2012- a TRS 090 for 134A...supplied by Alpinair-www.alpinair.co.uk...in W2 central London- cost 299 new back then. I think the S means 134a gas (See Barrys post for the implications of a gas change) rather than the F being R12..no longer available.
This pump is also an Imprezia part (with the right pulley) - there is one for 80 on ebay right now- but its in Malta, 25 postage, or a Shogun one with the wrong pulley in Essex. I'd go for the Malta one.
Here is a new one..but you need to call for availability.
http://www.apairltd.com/product/13539/1 ... rs090,-12vIf you want a new pump, having had a quick look there are fairly standard mountings for Ac compressors...side (A610) or end (GTA) with side or end pipes and either poly V (610) or classic single V belts like the GTA so a number of different ones could be made to fit the engine. The worst case would be having to modify the connecting pipe.The other thing to look at is their pumping capacity...that is set by the number of pistons they have.
I found a catalogue that sets it out...but the Trs090 seems to be easy to source so I'd go for that.