In years gone by, I made 5 Trans Atlantic crossings in light twins and 3 in single-engine aircraft.
The rules posted by Nicolai pretty much cover US registered aircraft and offer general GA operations in that area. For other counties of registrar, obviously you'd have to check with them.
For flights FROM the US which transit via Canada; the Canadian Authorities USED to require specific types of equipment be on-board, and would require the pilot to meet specific experience requirements. Albeit, that was several years ago and those rules may have changed.
There are a lot of variables which contribute to a COMFORTABLE journey across the pond in a Light Aircraft. Far too many to arbitrarily address here. PM me if you have specific questions.