I bought this nice old hollow-body Diplomat guitar recently - apparently they were made in Japan in the late 60s. I got it fairly cheap.

I'm thinking it might be nice to replace the pickups - it has quite a boxy / twangy sound, a little like a banjo. Or do you think this would be more down to the shape / wood? The body is quite shallow.

I'd like to buy some pickups off Ebay, since I live in New Zealand and this is usually the most affordable option. I'd like a nice bright trebley, jangley, slightly harsh/dirty sound, but not lacking in low-end... something a bit grunty? I'll be playing mostly clean-ish, but may want to crank up the gain occasionally, maybe to achieve some kind of post-punky sound vaguely in the vein of The Birthday Party or Gang Of Four. I think single-coil pickups would be my best option?

What pickups would people recommend?

On another note, the bridge is an ancient design which doesn't even have adjustable intonation, so the intonation is quite badly out. Would it be a terrible idea to buy a cheap bridge on Ebay and install it myself (given that I have quite a bit of DIY experience)?
Take a look at Lace Alumitones. I put some in a hollowbody, and have been generally pleased with the results.
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You could throw in a set of Les Paul type humbuckers. I have a Conrad guitar of similar construction that I won on Ebay and brought back to life. Changing pickups on an archtop is a difficult undertaking. You will have to change the pots out also not just change pickups.
I'd start with a tailpiece and bridge change. If you can live without the whammybar, a simple archtop tailpiece sized for a thin bodied guitar, and an archtop TOM bridge that can get low enough to match the bridge height you have now would improve tone, playability and intonation.
Don't loose the groundwire when you remove the existing tailpiece. There's a wire that needs to be reconnected to the new tailpiece to ground the electronics. If it slips back inside the body out of reach, you're screwed, so be carefull during disassembly.
Good luck.
Thanks for the responses!

Why would it be necessary to change the pots? Would this just be if changing from single-coil to humbuckers? I reckon I'll stick with single-coils, I like that sound, and already have another guitar with humbuckers.

The guitar's electronics are actually mounted on a removable plastic plate... so perhaps this guitar would be easier to tinker with than most hollow-bodies?

Why would changing the bridge mean I'd have to change the tailpiece?

I looked up archtop guitar bridges... this seems to mean a bridge mounted on a wooden base which is arched to match the curve of the guitar... What's the advantage of this, out of curiosity? Would something like this be unworkable?
Archtop guitar bridges are adjustable for intonation by moving it up and down the strings. They are held in place by string tension. Get one that matches your fretboard wood. If you're keeping the whammy, get an archtop roller bridge to help keep it in tune.
The bridge you show requires drilling two holes in the body at just the right place and is built for use in a solid body guitar. It won't work. There's no blocking in place to hold the studs.
Changing out the tailpiece might give your guitar a better look, tone and tuning control, but isn't necessary to change.
The capacitors used in single coil pickup electronics are different from those used with double wound humbuckers.
Ah thanks, that makes things a lot clearer.

So back to pickups - I'm not looking to spend much at all, and hopefully buy used, since the guitar was quite cheap and I don't have sky-high expectations from it. Are there any particular plentiful used pickup types that I might find on Ebay that people could recommend?
As I recall, the single coil pickups are wider than humbuckers are. So the existing hole in the guitar might be too big to be covered by the humbucker plastic when installed. I had to make plastic flanges to go under the humbucker's mounting to cover the holes in the body when I put humbuckers in mine. Good luck.