You have to pull the plugs, no other way around it, you can either get away with just cleaning the holes with a thread chaser or like you said, helicoil, but look at it this way, if it's not the plugs, you will have to pull the head anyway cause it's probably a valve issue. Wouldn't think so at 3700 hrs, we've had some luck by slowly working the plugs in and out while soaking with penetrating fluid , getting a little at a time and thread chasing the holes, and of course applying anti seize on the new plugs,good luck