Should be fine, I used to use an old Evinrude 2hp and it was ok. A longshaft would be bulkier to handle, heavier and a pain coming into shallow water, slips etc.
Traditionally you used a short shaft Seagull on your Mirror but oars were a must . Also a good idea to cut a short centreboard to go in the casing. I know a merc3,3 2 stroke works as well. A copy of mirror mania on board of course.