There is not much in the way of actual repair manuals for MTD mowers.
Not having done that particular model I should have done a full frame check before writing my post.
Checking the frame page
https://polr.partstree.com/v1/iplimage.php?ir=YToyOntzOjk6ImltYWdlUGF0aCI7czoyNDoiYlhSa0x6YzJPVEF5TXpRelhEQXdNRFE9IjtzOjc6Im9wdGlvbnMiO2E6MTp7czo1OiJ3aWR0aCI7aToxMDAwO319
It is apparent that the fasteners you need to access are bolted to a plate that is under the main frame not above it as earlier thought so there is no access from the top of the mower. Sorry about the misdirection.
These are an even bigger PIA to access
By now you should have the deck off, is not do it.
the easy, but complicated way is to unbolt the plate ( which does not seem to have a part number ) from the side rails of the frame there are 4 or 6 self tappers holding it in place.
On some it will drop down a few inches if the gear on the bottom of the steering shaft ( # 11 ) is removed first.
To get it down further you usually have to remove all of the control pedals and you want to avoid that is possible.
This will drop it about 2" at the front so you can get at the heads.
Downside is the target fasteners are now not supported so you need a rattle gun on one end and a ring on the other.
The other way is to slip your fingers between this plate & the frame.
You can feel the fastener heads but will need a 0 offset ring spanner to get on to them.
The bannana or cresent shaped ones work best because there is not much room.
Don't even think about using an open ender. although I have resorted to using a flare nut wrench on one mower because the spanner was too thick to pass over the top of the nut & under the frame.
The bolt ( # 1 ) is captive if the plate is not dropped so it can not fall out.
To replace the nut, which is close to the open end you can jamb it into the end of the spanner using tissue paper or cling film.
I have done some with needle nosed lock jaw wrenches to get the nut started.
With a lot of these type of jobs I stand the mower up vertical because it is easier working sitting down in front than laying down underneath.
If you go that way, put cling wrap over the fuel tank outlet the do the cap up tight ( or drain the tank ) & remove the battery to prevent messy spills.