Hmm, this is a bit of a tough one.
#1 - Yes, but only if its approved by DTEI (aka regency pits) and signed off by a registered engineer. Getting it approved will likely cost $5k+, and they will probably require brake upgrades too. The up side is it'll be fully legal, so the cops cant defect you for it.
#2 - Best and easiest are not the same thing. What is best depends on your target use for the car, and budget. Any idea?
#3 - Not sure, but i very much doubt its worth the effort to find out/try.
Because the SA63 was pretty much the last of the carb'ed cars, and you cant ran an engine older than the age of the car, all options mean moving to an EFI fuel system (lift pump, swirl pot and high-pressure pump).
They also require more wiring work.
The easiest option would be installing an engine xmember and 22R-E from a RA65.
It'll be a decent amount quicker than the 2SC, but it still wont be 'quick'.
You should be able to do this upgrade legally, without any engineering required.
Next best would probably be a Blacktop BEAMS 3SGE import engine from a Lexus RS200 Altezza.
Im not 100% sure if this will work with the SA63 engine xmember or not (may need some mods). Some research is needed.
Get the engine with the 6 speed gearbox.
This engine & gearbox would make the car a lot of fun.
You might just be able to do this legally without any engineering. Submit a request to modify form to the DTEI to find out.
Expect this conversion to cost you $5k+ in parts incl the engine.
After those two, it all depends on your budget.
Pretty much anything quicker than the BEAMS 3S engine (ie turbo anything) will require engineering to be legal.