Drill exhaust manifold bolt - Remove head or not?

Yes remove head and find a small tool and die shop that does repair work on tools and dies as in die cast dies and/or plastic injection molds. Have their most experienced journeyman cut out the screw with a carbide cutter on a Bridgeport type vertical milling machine. Express how rare and expensive the head is and not to **** it up. Guaranteed they will have someone that can do the job as it is common practice. Make sure you express how you want it done with a milling machine, clamped to the table.

Welding nut or whatever or even using a hand drill is hack work likely to **** something up.

Then as RRR suggests, time to service the heads, valve job, fresh springs and such. You won’t regret spending the money for a job well done the best way.