Gonna be cheaper to replace the motor with a 2nd hand jobbie I think.
If its knocking it will need a set of shells at the very least. If its only been for a few secs, then you might get away without damaging the crank, else it will need a regrind / polish and oversized shells (a reground crank is never as good as a virgin crank though, and is more susceptible to future failure again
You then have to meticulously clean everthing, especially check for any damage in the oil pump if there is a lot of swarf in the sump.
Pretty sure your piston rings wont really be effected - they are not something that requires oil pressure to keep them lube'd - just the oil splash from the sump.
...though, saying replace the motor is easy for a common engine, just realised its the C20XE which is the redtop?? and getting quite rare?
Drop the motor and give it to an engine shop the inpsect / rebuild - at least then you know its going to be like new. Id expect £3-500 depending on how far you want to go with new parts - id certainly consider new oil / water pump as a matter of course if the engine was out.