If there's no stud protruding from the head surface, you can drill the stud out and (hopefully) just use a new stud, not a Helicoil.
Use a spherical rotary burr 1/8 inch diameter on a Dremel tool and put a dimple into the broken surface of the stud. Keep the dimple as close to the center as possible, working it around the center until the dimple is on-center of the stud. Then use a hardened carbide drill-bit less than the 1/8 inch diameter, say 5/64th. Drill completely through the stud. Follow that with a 5/32 drill, then use an Easy-out reverse fluted extractor. There are different grades and types, the spiral fluted case hardened are the best. Before you try and rotate teh extractor using a T handle, hammer the extractor into teh hole you drilled. Hammer it good and tight.
If that size doesn't work, move up in drill bit size another 1/16, then use the next largest extractor. Don't go above a 1/4 drill, as you'll cut into teh threads in the head. Lots of penetrant and cutting oil too.
I've done this to countless B18/20/21/230 heads and never had to take a head off and use a Heilicoil. Be patient, go slowly. It's tedious to do looking sideways into teh broken stud/hole, but should work fine and not need much else. Often the stud has been so frozen that even after drilling out to the largest extractor size possible, I've had to use a sharp strong dental pic to pry out the pieces of the remaining threads. A that point a fine pointed 30 degree burr on the Dremel can cut a groove in the side of the remaining stud, collapsing the stud, then removal is easier.