Welcome to the forum.
I know the west coast of the Hebrides so completely take your point about a back up engine. You've got your fair share of fishing flotsam, obligatory lobster pots (sometimes poorly marked), unpredictable weather and of course a lack of other vessels in the area for peace of mind.
It's possible to squeeze an auxilliary on the transom of a Searider 5.4, but a lot depends on you main engine lump size, if an A-frame is fitted, etc. A slimline 2-stroke auxilliary should fit.
One piece of advice if you're fitting to the transom is to add a stainless steel eye on the deck to tether the auxilliary while in the raised position. Most of the smaller auxiliaries just aren't built to be on the back if a fast-moving RIB so vibration can be an issue.
My main piece of advice is to run the auxilliary every time you're out and of course prevention is better than cure so main engine should be serviced meticulously, inline fuel filter added and the usual visual checks before heading out.
Earlier thread on the topic here: http://www.rib.net/forum/f8/auxillar...-up-41304.html