1) I don't believe in the cost factor, last time we did a swap for our Van (CR 27 with 2C engine) it costs less than Overhaul. and mind you that with the new (Used) engine you get new power steering pump, A/C compressor, Alternator, fuel pump, Transmission (if it included in the deal) etc.. even with the overhaul you never know what kind of parts were used to overhaul (Bearings etc..) and you cant still get the original specs even after a overhaul.
2) You can re-register the new engine number at RMV.. we did that with our Van.. (It's not illegal after that)
3) That depends on the place where you get the Engine.