Just a thought
I considered something like this on my Cal 27 just in case because I carry a 6hp for my dinghy. However, if the engine fails then you can always sail.
In the event you can't sail because you are in a channel or something and trying to buck through chop and headwinds think about how you are going to reach down and lift that heavy thing down to the water, somehow contort your body enough to pull the cord the numerous times it would take and then adjusting throttle...etc all the while you are in chop or current.
If a current is pushing you towards rocks or a bridge, would you have time to even get it moving? And what is a 4 or 6hp really going to get you on the tartan 34? Maybe 3 kts or less with the outboard popping in and out of the water in the chop.
Maybe instead of taking the time and energy and money of mounting an outboard, you could put in a new carborator, water pump, starter, ignitinon, plugs, exhaust, fuel lines, dual filters, upgrade batteries, or whatever it is about the motor that keeps you up at night, and really get that motor back to reliable condition.
Just my .02.