Floor Delamination - small area

[FLexpat]

I have no idea what injectable solution is best - my sponge-core is way too big for that.

That area is directly above the stringers on my 76 (also an I/O). If you pull the fuel tank hatch you can see where the stringers are forward of the engine well. If you look from the engine well you can almost see where the motorbox hinges go into the deck - you may be able to get at a little of it from the engine well.
I don't know if this will help you or not but here is how that part of the Tsunami/Sceptre is built -
Original deck schedule for early ‘70s 23’ Sceptre; top to bottom (from Feb ’73 Boating, Boat Test #207)
1 layer 3/4 oz mat
2 layers of 1 ½ oz mat
1 layer 22 oz roving
1 layer of 1 ½ oz mat
3/8” end grain balsa
1 layer of 1 ½ oz mat
1 layer of 22 oz roving
Underneath that is about 3/4" of 'Potter putty' then you get to the stringer shell which is about 1/4" thick roving. The stringers span from the inner to the outer steps; directly over the middle panel there but they have a rigging channel in them about 3" deep that might be under the outboard part of your soft area.

Good Luck - Post pics when you get into it.