Chainsaw Repair

I'm working on restoring an old Husqvarna 385xp thats backfiring.

Ive rebuilt the carb redone the ignition and now that its still not changed i'm looking at bigger and uglier issues
 Based on how dirty it was I'm trying to pull the exhaust and look at the cylinder (I also half suspect the muffler may be clogged up based on how dirty the saw has been and that is a possible source of the backfires.

Problem is the two deep bolts are so dirty that i cant find the hex head, nor do i know the size of metric allen key I should be fishing in the hole with.

I'm hoping someon knows the bolt size off hand or if there is a trick to getting it off other than the sawzall or angle grinder if those bolts are seized or stripped (Or is that time for a new piston and exhaust?)