18hp Briggs runs hot and lacks power under a load?

18HP Briggs & Stratton (in a Troy Bilt) loses power under load and runs hot.
Model- 31D777 Type- 0233E1
This engine has had the head replaced on it last year, thinking that it had a burnt valve after it sheared the key. It seemed to run fine after the head and key were replaced. I noticed that at the end of last summer it seemed to be running hot, the pipe and muffler glow red after a few minutes of running. I replaced the muffler at the beginning of this year because the old one was burnt out. It has now burnt the new one out.
The first mowing this year it started running rough, no power when under a load and backfiring. It had once again sheared the key. I replaced the key and now it idles and goes through the power band OK from what I can tell but it doesn’t have good power when I turn the blades on. With the blades off it sounds and runs fine but still gets hot. If you go though a little taller grass it falls on it’s face and you have to sit there and let it build power. Or if you are going up a grade it struggles to get up it.
I have taken the carb off and cleaned it although it appeared to be OK. The plug has a gray/white look (running lean) at the electrode but has black dust at the base (like it’s running rich). I’m lost on this and need help. Needless to say I don’t think that the head was the problem last year. ?