Essentially, you'd need to keep the Revolution from happening, and then split up British America into a Northern and Southern dominion at the traditional free-slave lines (Mason-Dixon Line, Ohio River, etc.). Obviously, that gives Canada more than just the northeastern American states.
By that point, however, this Northern Dominion is far less Canada as you'd know it, and far more the Northern USA in cultural terms - just with a British monarch as head of state, and that happens to include OTL Canada within its borders. Precisely because the "Canadian" national identity wouldn't develop as an us-vs-them scenario vs Americans, and thus the settlers (who WON'T be in a mass movement and self-aware of why, but instead a far more gradual colonization with no greater reasoning than "free land!") moving into OTL English Canada would never consider themselves as anything but American, the way Ohioans and Indianans do. The vast majority of "Canada" as you propose in this scenario would be living in the area of OTL America, tilting weight and culture and economy to those areas even if *Canadian provinces will be prosperous as in reality.
Your best bet for the northeast is just northern Maine, maybe eastern Maine too if Britain can grab it in the Revolution and combine northern/eastern Maine into the proposed New Ireland.