I wouldn't let the fact that it is over TBO scare you. While the value of a 4000 hour engine is certainly going to be less than that of a 3000 hour engine, the value doesn't automatically plummet when it goes from 3,599 hours to 3,600. As long as the engine has been properly cared for, it will still have lots of life left in it for a part 91 operator.

I have several thousand hours behind PT6's that were beyond TBO (some with well over 10,000SOH) and have never had an in-flight failure. With proper care and active trend monitoring, you should have no problem getting plenty of life out of that engine.

I don't believe there is any insurance policy that will pay out if your engine suddenly needs an overhaul or reaches a life-limit (unless, of course, the airplane just 'happened' to catch fire the day after finding out that it needed a major overhaul).