Cleat Help Needed

In real the old days, machine screws were only threaded part of the way up from the end and this threaded end was larger in overall diameter. Past that to your cleat, thru the deck, was slightly smaller smooth rod (shank).
At the time, it was thought maybe better shear strength.
Under that deck is a threaded back plate aweigh from the larger hole end of those slots.
Once you verify my thoughts, plan to re-use that back plate, a few inches below the deck surface.
If the original cleats are not available, I would fill all in with epoxy, make a template from your new cleats, Drill and tap per that pattern and secure down some big asp cleats.
Your going to need long taps.
Hope this pans out and helps.

Could be they use those round head nuts? (Acorns)

Ben thinking about this all morning.
I do not see any threaded holes in the next lower plate.

Scratch my precious post.

This is not your typical thru bolted cleat at all.
Then Dark Horse got me on the correct trail.
Carriage bolts pointing up with barrel nuts.
Carriage bolts and acorn nuts will probably be your simpler solution.

These will hold the plate your cleat is attached to.

Lots of TeffGel where the carriage bolt comes thru and makes contact with the alloy deck.

Barrel nuts come in stainless. This can look like a screw or security screw when viewed.
https://www.mcmaster.com/products/b...els-and-screws-1~/material~stainless-steel-2/

Welding on could be a fire hazzard from the inside.
Drill and tap,, Heck yea. Tap both the deck and that plate rite below it.
Make a derlin plate under your stainless cleat to reduce corrosion. TeffGell on the screw threads.

You may be making a larger plate to cover those slots if your new cleat is not large enough.

Any pictures of your old cleats? Maybe from another boat.
Start e-mailing those pics out to all the salvage shops you can find.

Use large socket head machine screws (bolts) Thread nuts on screws but do not tighten.

Insert in large holes, slide to the end of the slot. Pull up cleats or screws and tighten with and impact driver.

No need to get under the deck to tighten.

Sometimes this works with an impact wrench but not with a hand-held ratchet. Depends on whether the bolts will grab the bottom of the deck.

Cleats would need to have identical dimensions.

Is it possible there are corresponding slots underdeck to hold the nuts?

You could also insert the bolts head-down. Pull up on screws, hold the screw with locking pliers. Cut off excess with zizz wheel.

make sure you put some isolation tape or similar to avoid dissimilar metals with stainless against the aluminum. Certainly not as bad as other metals but will still react. And I would not use an impact when bolting it up as they tend to gall threads on stainless.

And use TefGel on the stainless threads too