Sorry I meant to add that you could also use an epoxy mix to secure the plate to the transom, and then add an epoxy layer or 2 to the whole assembly to ensure a waterproof and physical seal. Your outboard would disintergrate well before anything weakened on the transom, and it would save you tampering with any of the buoyancy designs on the original boat.
No, Epoxy does not stick to aluminum nor does it stick to Stainless Steel. It also is far too brittle and will tear under motor loads and vibrations. Further more you need to protect it from ultra violet (UV) rays (sun light).
Covering the entire bracket is also a bad idea. It will compromise its function which is that it allows us to remove the bracket leaving only a small shoe on the transom. Once the shoe is covered by epoxy the bracket will not fit properly anymore.
As for waterproofing you need something flexible like Rubber or Sikaflex. The dynamic forces and vibrations created by the engine will constantly move the bracket relative to the transom, no matter how tightly it is fixed. Hence we need something that is flexible enough to guarantee the bond between bracket and transom stays waterproof for many years, yet flexes with movement. As an added bonus Sikaflex (or any other brand of PU based sealant) does bond reasonably well to steel, aluminum and very well to GRP. But Sikaflex alone will not be strong enough, you still need the four 8 mm nuts and bolts and a backing plate.