I dont think there is a regulation preventing having a starter motor, they simply dont have them due to weight saving and also with stall avoidance systems we rarely see a car stall now but..
1. With the old KERS units that directly fed onto the engine crankshaft could it have been used to restart the engine.
2. I believe the MGU-Ks dont have a direct input onto the crankshaft like the old KERs did and think they provide direct drive via the gearbox or rear axles (could be wrong) and so could this be used in extremis to restart an engine (like a bump start)?
This idea/question came to me after the double MGU-K failure of the Mercedes which they couldn't reset. I wondered whether a full shutdown/reset of the system using their energy store may have brought the system back to life. Basically a 'Ferranti Reset' as I call it.
Superficially, it seems that function could be achieved by the MGU-K. All the parts are there.
But the questions is, are they the right kind of parts? A starter motor is a series-wound motor that is geared to the crankshaft. Those series-wound motors generate tons of torque and can spin up until they self-destruct. They also generate a lot of heat.
I really doubt if anyone in Formula One uses a series wound motor as the MGU-K.