Permits are not mandatory for private flights, but if you plan to land, it is essential to send a General Declaration (GenDec) to customs in advance and confirm its receipt. Additionally, the Pilot in Command (PIC) should make a phone call before departure to ensure that customs is prepared for the flight.
Commercial flights require more extensive preparations. You must apply for a Canadian Foreign Operator Certificate, which should be completed several months in advance. However, in any situation, American Flight Support has a satellite office in Toronto and can expedite the process of obtaining all your permits swiftly.