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.