Probably the ultimate power to weight aux is the Tohatsu 9.8hp 2-stroke. I have one for those attributes as a main engine on my small Zodiac.
At around 26kg it weighs a fraction less than a 4hp Mariner 4-stroke. A no-nonsense design with no techy stuff to go wrong and a good name for reliability. Easy to start and quiet for a 2-stroke. Ideally the standard prop will need changing to push a heavier boat at displacement speeds.
I have one of those sprung aux brackets on my grp boat and even with a 3.5hp it takes a fair bit of stick when nipping along in the bumpy stuff. If there was any possibility of a strong fixed bracket or transom mount I'd go for it.
Have a look at this thread too on failing aux brackets...