言ったら is the conditional form of 言う, so it means “if you say/name”. So literally I read the phrase as “what(ever) if you name a reward is good” or “Whatever you want in thanks is fine”.
EDIT - I read @buburoi’s reply, and I guess my answer doesn’t include the か at the end of the sentence. Maybe it’s better translating “What if thanking is good?” or “What can I do to thank you?”
かつやく is a noun mean activity that can act as a する verb meaning “ to participate actively; to play an active role”.
I initially thought the し was the stem form of する, but actually there is a verb しはじめる which means “to begin (doing)”.
So this is “he began doing an activity” - the activity itself then being described in detail just before it in the sentence.