Scotland would likely need to:
Join Eurocontrol, join ICAO, and develop a relationship with the Single European Sky, whether in the EU or out. Scotland would also need to establish a CAA and hence a safety regulator with all that that means and to appoint an ANSP for en-route services, which is likely NATS or more likely SATS a new Scottish ATS Employer.
Then all the UK agreements for ATC would need to be re-visited.
Or... Contract with the UK and pay for all of the above as an outsourced service.
Not a trivial thing. Years of work for consultants for sure!
Then what about aircraft registry?
Interesting times.