Working on a miller welder/generator with a Kohler. Found several issues but the one I’m looking to address currently is on the cylinder head. Where the muffler attaches to the head around the carb side bolt threads it’s all “washed out” by exhaust gases I’m guessing.
Could I repair this with some steel putty?
Is this a sign of a blown head gasket?
Is this a sign of issues with the exhaust valve?
The muffler mounting bolts were really loose

Is this pic similar to what you have:
https://www.partstree.com/models/cv12-5-1211-kohler-command-pro-engine-made-for-cub-cadet-12-5hp/exhaust-11-27-209-tp-2351-5/ 
 
Are there stud's sticking out (or supposed to be) of the head?
 
Your question kinda bounces around a bit..
 
Please clarify, is the issue just between the muffler not being tight to the head or???
 
And a full ENGINE MODEL # would help us to see exactly what set up you have..  
 
.

Having issues posting pictures but the issue is not with the muffler, it’s with the cylinder head where the muffler mounts to the head. It’s missing metal around one of the bolt holes. Kohler Ch12.5S I believe is the model number. And no studs, bolts go through muffler and into the head

Ok, that makes sense..
 
Being about the hottest part pf the engine, JB Weld very likely won't work.
 
Your best bet is to have the missing part (aluminum) TIG welded putting material back
where it's gone.  
 
That, or a new head.  A new head ain't cheap:
https://www.partstree.com/models/cv12-5-1211-kohler-command-pro-engine-made-for-cub-cadet-12-5hp/head-valve-breather-4-27-206-tp-2351-rev-9-6-2006-9/ 
 
You can probably find a used head on Flea Bay...
 
You know what you have, a good welder can fill that cavity in.  Of course it may need to be re-drilled and 
re-threaded.  
 
 
.

Yea I’m not to sure bout the JB weld idea either but I did buy something they make called extreme heat. It’s like a hardening epoxy paste product that is supposed to be good for 1000 degrees and lists exhaust’s as something it’s used for. May try it for chits and giggles. I can’t weld aluminum and would have to take it to someone for that repair. Mainly trying to get this thing running to verify power output from weld leads and the 110 receptacles before diving any deeper or sinking much money into it. It’s 1995 Miller Bluestar 180k with about 700 hours on the clock

Ah, ok
 
Also, if you can put studs into the head, if enough room around the muffler, 
would make a stronger point- 
 
IE, no threads in the head having a bolt turning thru it.  The stud is stationary, nuts 
after, to pull the muffler in...
 
Good luck and plz post back how it goes..
 
.

Waiting on a couple parts for the carburetor and a new exhaust gasket. Re sealed the OHV cover today with the right stuff grey. Will update on Thursday when I attempt to fire her up

Got it semi put back together last night to attempt to run it and check power output from welding leads/receptacles. It still runs the same, which is starting to briefly then backfire out the intake and possibly the exhaust as well. I’ve pulled vakve cover and checked rockers and push rods prior to this so pretty sure valves are not the issue. Believe it’s ignition or out of time. Gonna check spark plug and coil but I’m pretty sure I’m gonna find out I have a sheared flywheel key if I pop the flywheel off. Will report back with my findings