Here's a helicoil kit that I used. The rockers and exhaust are the same size, M8 1.25.
Heli-Coil - Metric Thread Repair Kit Part # 5546-8

LINK TO KIT Heli-Coil 5546-8 - Metric Thread Repair Kit | O'Reilly Auto Parts

I tested this out on an old 317 head that I had before working on the PRC 2.5 head. If I ended up drilling too far or breaking off a bit or tap there were no worries.

The O'Reilly's kit and part number are above.

Here's the bit that used since one was not provided in the kit. 21/64 bit

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The right side was not drilled far enough. I went about 1/2 inch down, and this left a few threads of the helicoil hanging out. On the left side, I bottomed out the drill bit in the the original hole. I sunk the helicoil about 1-2 threads down from the surface.

The red dye is from me doing a dye pen test to check for cracks. I can only assume that the rocker bolts were rattled on with an impact or just flat out over torqued.

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Drilled, and retapped
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showing the tab at the bottom
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here's the tang removed. I blew it out with a rag and brake cleaner with a magnet to capture it.
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nice work