I had Exactly the same problem , I already had bought a 4 stroke 4hp Yammaha before I bought my Rib , When I came to fit it as an Aux It would fit "JUST" but couldnt turn the main engine.
I Figured that the Idea is that it is an auxillary engine"For Emergency Use Only" and that the operation of the main engine was my primary concern, so as long as that had room to work properly I would be prepared to change the aux for a smaller 2 stroke.
I then found the same problem, There are no brand new 2 strokes on the market now.
I decided to get a used 3.3 Mercury/Mariner/ Tohatsu or Johnson as they are all Exactly the same just painted different colours. but are very narrow and will fit on my transom with the main Engine.
I ended up with a fantastic Mercury 3.3 , In excellent Mechanical condition , obviously had little used.
I did change the Impellor , spark plug gear oil and shear pin just to be sure.
The main drawback is NO reverse gear !! , But on a lake or the sea with no significant tide will push my 5.5m Humber along at 4+ knots forwards, and can steer It enough to get me out of Trouble.
However I have to use the main engine to sucessfully steer the Boat if on a river with any flow or any tide flow.
I have made a point of trying the Aux in Various different real life situations , just to be sure it will perform when its needed to.
Always fulled , tested and run up to temp before leaving on a trip anywhere
The main advantage is its small and lightweight (lift with 1 hand ,less weight on transom bouncing around on trailer)and no mods needed on the transom or engine.
Hope this helps !!