Any decent bolt supplier should do these cap heads, just don't use SS ones.
I would either try grips, if enough to grip hold of, or failing that as said chisel.
When you have fitted the new ones, fill the spline/hole with gasket sealant, this goes semi hard after a few weeks, protects the head from corrosion, so next time just dig it out and spline fitting will be fine to use.
tabetha