I would much rather use self-repairing. It wouldn't be too hard to fix a tiny fenris engine yourself, would it? I am going to make it so that you will have to hit a key to make the turret fire at the target, but if you want the turret to fire upon a different object, you will have to hit that key twice. This allows you to use the same key to turn it on and off, the first tap makes it fire on one thing, but the second makes it fire on every thing. For comparison:
Ship/Keys Used/SEXPs Required
Fenris / 6 / 29
Aeolus / 8 / 39
Deimos / 12 / 59
The SEXPs are just slightly modified duplicates of the parent SEXP, so it shouldn't take too much time. Good thing you don't fly a Deimos often. You fly corvettes in the last few missions, which takes place during the Capella Incident.