Exhaust stud hole needs help

Hi all,
I have an exhaust stud hole in my cylinder which is about half gone.
I removed the exhaust stack and part of the helicoil is showing and upon
further investigation, it looks like about half of the threads holding
in the helicoil are gone. The stud and the nut came in one piece. I
removed the helicoil since it isn't doing its job anymore and I'm
curious about the next course of action.

The helicoil is a 5/16-24 and it should have been a 5/16-18.

I suppose I could put in a new one and hope the remaining threads will
hold the coil and stud, but I'd rather fix it better.
I'd also rather not pull the cylinder and have the hole filled and retapped.

Any good suggestions out there? Some sort of threaded insert?


Thanks

Dave