Which gauges are compatible with a Subaru engine & gearbox?

It's not all that involved, really, since the gauges, themselves, don't give a hoot WHAT the engine and transmission are.....all they care about are the senders on the engine or wheel end.  See Carey Hines at Special Edition (the Beck folks) for a gauge set, but bear in mind that the 356 set might be larger diameter than your 911 gauges.

The Speedo gets it's speed signal via mechanical cable from the left front wheel.  Nothing changes from VW to Subaru.

The fuel gauge gets it's signal from the VW sender at the gas tank - again, nothing changes.

The Tach gets it's signal from the "-" side of the coil and that may vary depending on what you're using to fire the ignition.  If you use the more popular aftermarket EFI systems (or the original Subaru and for that you'll need a hacked chip set to make it run) then there's a port on the EFI computer that provides the proper signal to your tach.  

The temp gauge and oil pressure light get their signals from VW senders mounted to the engine case - you could use the same senders and use the existing Subaru oil pressure port on the side of the engine for both, although it might take a little fabricating or adapters to get the threads to match.  Folks in the world of Dune Buggies have had this engine/gauge sender thing licked for years.

David Stroud or Peter Venuti on here could probably help you with this, too......

Ossie,  Henry from Intermeccanica had Hartmut from North Hollywood Speedo make special gaiges for his Water Cooled cars.  I have them in my Subaru IM.  I think Peter's car was built before these arrived.  Here is a picture.  They are expensive.  The word at Carlsile is that Henry and Carey are working with VDO to get new Gauges made for mutil-use. 

If the Subaru computer and harness are pre OBD II, you won't have to hack the chip. The motor doesn't care what ECU you use, but the sending units and cam and crank sensors MUST be compatible with the harness and ECU(best if they come from the same car). You can make your own harness too using the original and stripping out what you don't need. The Suby computer has a tach out which should work fine with whatever tach you have, as long as it's a 4 cylinder tach.

You will need a speed sensor though, the Suby computer likes that. This can be rigged up with magnets and a sensor. A lot of Vanagon/Subaru guys retrofit the O2 reset box with magnets and put it in series with the speedo using two cables.

Thanks guys for all the good replies.  I was planning to use the ECU and wiring harness from a donor Subaru if can get hold of one as I was aware that made everything a lot easier.  There's a guy here in Australia who can apparently modify the hardness to work in a "stand alone" configuration.

Doesn't sound like the gauges will be too difficult either way and I have a speedometer repair shop nearby who can assist if needed.  I'm doing a left to right hand steering conversion at the same time so doesn't really matter about the size of the gauges as the dash has to be rebuilt from scratch anyway.