That is a hard call to answer as we do not know how hard it is to gets parts for the LS1 engine in your country and what you could spend.

Maybe do a compression and leakdown cylinder tests and see how the rings are doing. If still fine then maybe at the least pull the heads and redo the valves and springs.

At the least even with not taking heads off is replace springs, check pushrods are true and same for rockers.

If needed replace what you can without even taking heads off.
I'd really test the fuel injectors, sparkplugs and plug wires.

If you yank engine, then flip a coin as to a total rebuild but best is swap to a LS3, stronger engine and suited if going boost later on.

Here is a 2008 C6 of Melvin's who is at the USA base in Germany I just tuned this weekend

He was able to pull 192 MPH on the highway and gained about 50 HP with just a tune so LS3 is a strong engine