if you catch it really early, you can get away with just slapping in a set off bearings. We did it to a mates car, 30,000km on and its still strong.
If you've done a bit of damage to the crank you may have to get it machined, so pretty much a full rebuild will get done if the motors tired.
or like you said just buy a 2nd hand motor, probably the cheapest option, but you may be buying someone elses problems
OR
slap a rb30 bottom end on your head