stuck exhaust valve bolts

Hi.I have a 2004 polaris 600.Im trying to service the exhaust valves but have found the two bolts that attach it the cylinder to be set up.I tried to turn them on a warm engine.
Any ideas welcome.Im also looking for10mm 6 poit wrench as the bolts also look to worn for a 12 pt.
thanks much
john

stuck exhaust valve bolts