Jlam,
You are correct that "except" generally introduces a necessary condition, and it does in this instance:
"none except the more virtuous deserve praise"
"Except" precedes "more virtuous." Therefore, "more virtuous" is the necessary condition. "Deserves praise" is the sufficient condition.
DP
MV
Be wary of veering off into exercises of rephrasing conditionals using constructions other than the standard "if... then..." because you can wade into a minefield that way.
You would not rephrase this statement as "the more virtuous deserve praise." Rather you could write: "Those who deserve praise are more virtuous."
Your construction "Only the more virtuous deserve praise" is correct.
"Only" can introduce a sufficient condition with the syntax "the only" as in "The only people who deserve praise are more virtuous." This is because the word "people" refers to those who are more virtuous, a condition necessary to deserve praise.
Look, Jlam, don't complicate this unnecessarily for yourself. Start by recognizing the conditional language. Then ask yourself if something is required for something else. If you can't get an answer that way, ask if something is enough to guarantee something else. Try to deal with these conditionals without resorting to mechanistic translations of phrases you've memorized. Stick with "if... then..." and explain them to yourself in an easily accessible manner.
What's the requirement here? Gotta be more virtuous. What's that a requirement for? Deserving praise.
DP
MV