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I don't think cannons look good on anything, just get a proper flowing 3" mandrel bent exhaust and high flow cat and that should do you for your exhaust needs.
I have just that with twin 2.5" tips at the end and I reckon it sounds better than if it had a fart can on the end.
Engine rebuilds start at about 3 grand for an RB20 I think, though I'm not 100% on that. Depends on what you're after. If you want a stock standard rebuild then it's probably worth putting something better in there. RB20DET Silvertop engines from R32s are cheap and a good upgrade, they just lack torque.
I have an RB25DET in mine, I rekcon they're the best all round upgrade for a 31.
The swap to any other RB series engine is relatively easy, it's usually a matter of bolting it straight in onto the standard crossmember using standard engine mount, custom gearbox crossmember depending on what box you're using, and a custom tailshaft, again depending on if you're using a different gearbox. Wiring looms need a little bit of playing with as well. There's a few other little bits & pieces depending on which engine you're putting in. Try searching for engine conversions on this forum or the R31 forum and you'll get more information than you can read in a lifetime.
A popular upgrade for RB20s is the R33 or R34 RB25 turbocharger.
If you've got the single cam engine that you seem to think you have, then I wouldn't really bother putting any work into it and instead put something twin cam into it.
They're prone to rust in the A pillars because of the water channel design, like a lot of cars are.