1. A ply and glass box that does not leak is buoyant. There are many boats on the market that have in-built buoyancy of that type.
As I have said before, In my opinion in a collision most of the damage will be in the first couple of feet of the bow section. If you make this area separate then you must increase your chances of not sinking.
You could fill the closed off bow section with milk bottles if you don't want to spend £15 on foam. Or use any other closed cell buoyancy aid, Charles L favours bubble wrap.
If the damage to the hull is outside the bow section then a buoyant bow should prevent total loss of the boat and engine.
2. Marine grade 2 part foam from my supplier works out at about £5.00/cubic foot. 1 Cubic foot will support 27Kg in water.
So 3 cubic feet will float the boat, engine and other bits.
Sheathed in glass cloth, makes it a bit more rigid.
Soon be testing.
H-A