Just to add to the trim issue: I use a 60's vintage 6HP Johnson which drives my Commander as well as the 9 HP Johnson that came with the boat. It weighs 45 pounds. We pull the motor and insert the plug when we race. The trim is not as good when we leave the motor laying on its side in the lazerette. It makes a noticeable difference to haul it below into the cabin. I think moving it forward makes more of a difference than moving it low.

If you must do this at least get a much lighter motor. Worst Marine used to carry a 5HP Mercury that I've seen other Ariel owners use (although none had it on the transom). They were happy with that. Make sure you get a long shaft. Hung out that far aft you'll be spinning free more than you think when you get into rollers. You'd be surprised how quickly you can overheat in steady rollers when the cooling water is intermittent due to the shaft being out of the water as the bow goes up and down.