Issue is with your skill, not the kata.

The algorithm you've written is way too inefficient for a problem like this. The tests are deliberately written to fail solutions like these. Once you optimize your solution, it should pass in ~2 seconds. You're probably testing with small values on Visual Studio - the tests on Codewars will try massive numbers.

Only "siay" is fine there.

Expected:"igPay atinlay siay oolcay"
But was: "giPay nitalay siay loocay"
                             ^^^^

Not an issue, your code doesn't follow the specs.

your code fails to detect some smileys, e.g. ":-D"

C# Completions 5629

Are you sure they don't work correctly? I don't think so. Please read this before creating a kata issue and provide some proof when you create one.