I've had auxilliary engines on both my boats. Firstly a Zodiac Pro 420. Space was uber-tight, but I managed to squeeze a Tohatsu 3.5hp 2-stroke long shaft in. It was on a Plastimo adjustable bracket. Regards that installation - the Plastimo bracket used to rattle and I didn't rate it for such a small engine. In addition, the Tohatsu tilt push bolt is flimsy, so it must be secured underway. I fitted a stainless steel U bolt to the back of the bench seat and used a ratched strap which worked a treat.
My new setup includes a Mariner 4hp 2-stroke long-shaft. Cracking little engine and relatively light too. This time it's mounted on the transom of the RIBcraft 4.8m and fits like a glove. It's secured underway with a ratchet strap from the engine to a stainless steel U bolt on the transom leg.
I'd go for a 2-stroke water cooled engine every time, they're light, fit in the available space, etc.
Not sure what the Bombard transom is like, but if it's like other Zodiacs - it's probably got strengthening sections that restricts fitting the engine straight onto the transom. Got any pictures?
If I come across any suitable engines I'll post you a link.