Without checked bags, 90 minutes is sufficient time for an Intl-US connection at YUL. There is a direct transfer path that avoids you going through full Canadian immigration, and drops you directly at US pre-clearance.

However, what if your flight from IST is delayed and you end up with significantly less than 90 minutes? I'm not sure what "keeping the connection at your own risk" means, but if it means that if you don't make it, you're on your own, then I'd think twice. Walk-up fares to LGA can be pricey.