Even with a 2-stroke auxilliary with its own integral tank it's possible to decant fuel into the engine if you are worried about having to carry pre-mix. I run a Mariner 4hp on a 4.8 RIB. I always carry 2-stroke oil, so a quick glug of oil, and press the fuel connection and pump fuel with the fuel bulb straight into the reservoir.
Run the auxilliary engine as a matter of course, ideally before you leave harbour. I didn't use my auxilliary for a few months and found the impeller was siezed with salt. Salt water even spray is an issue. I also find the tell-tales on these small engines aren't particularly powerful, so flushing is absolutely essential, regardless of whether it's used. I now carry a piece of strimmer nylon in my tool kit for this very reason.
Ratchet the auxilliary engine down. If you're mounting on a bracket, then fit a stainless U bolt on the transom knee or similar. The tilt mechanism on the auxilliary so unless it's fixed in position when you're underway there's a good chance you'll damage it if you're out in anything remotely rough.
Couple of older but perfectly serviceable 8hp engines here: Engines (Outboard) items for sale at Andy Seedhouse Boats