Auxiliary Engine ponderings
If you search this site for 'outboard motor' or similar words (use the Google search engine) there are quite a few threads. Usually this question arrises with an E-27 or E-29. Hanging an OB on your stern is possible but would pose several engineering and usability problems.
Note: The E-27 was the only model designed with optional transom opening specifically for an OB.
If you have an original A4 engine that really Must be replaced (those simple little engines are extremely long-lived and also rebuildable) you do have some decisions to make.
I would advise you strongly and sincerely to take two aspirin and get some rest before entertaining such a thought.
Regards,
Loren