Usually the ships which tell you that their scan won't hurt have the pirate flag. You can find them very occasionally at nav points, but more likely from ssds or being interdicted.
I suggest not taking out a pirate hunt mission unless you were already intending to spend some time searching SSDs in one of the systems or bounty hunting at the nav point anyway. You can try carrying
cargo to try to intice interdiction (does that still work?).
These missions were probably designed for gamma 1 when you would get interdicted all the time when flying around slowly in system (which was great). Now interdictions are rare these missions are too hard to complete reliably.
Ive also had the ssd for the pirate counter offer before killing a single pirate, abandoned that (lets face it, its not clear what you are supposed to do) and got a hand in offer at the original planet which then gives you a plus rep blue arrow. Text stating well done for killing the bad guys, but I havent killed a single one :/