Depends on how long it's set with them being untouched. IIRC I had to drill the heads out of 3 on the one I fixed up. But it hadn't run in years. I'm not sure where I ordered the bolts. Stihl may still have em if your looking to keep it stock. If not you may want to try to convert them to t 27 torx.
Its a method call EDM metal disintegrator can move all broken seized screws -taps or whatever without damaging the holes or maybe and the threads