You need not be part of the same alliance but I would not count on any favors from ac at lga with two separate pnrs. I'd not want to have much less than 4 hours given delays at lga, potential weather, customs line ups or baggage delays. If you were arriving on aa you would need less time and might get some sympathy from cx as they are in the same alliance. Also, aa flies from lga to Yyz. For concrete transfer info check gtaa.com where it looks like you will need to clear Canada Customs regardless whether you fly ac or aa into Yyz.