Gday
Many years ago when we had trouble with Philips head screws or Allen head bolts in side cases or whatever on jap motor cycles engines we used a punch and a hammer , centring the punch on head of bolt or screw we then hit punch with several sharp blows
this would break the seel or grip the expansion and contraction the heat would put on the threads or oxidation on threads
it may well work for you on your motor
an impact screw driver was a good tool to but if not of good quality or if not used properly could damage heads and leave you with a bigger problem
just a thought yeah
when replacing bolts clean threads up on white wheel and apply anti rust lube or anti seize grease :O)