The Pay Pal currency cannot be changed currently in the "On" products.

The best Gateway to use if you are using CAD (Canadian Currency) is to use BBMS (Blackbaud Merchant Services) or Authorize.Net as both of these payment gateways accept CAD.

There is a existing Enhancement request for this functionality, you can vote for the idea here  https://blackbaudk12.ideas.aha.io/ideas/K12OB-I-578