Not familiar with riding a modern MachZ but I cant stress enough how important set up is. I used to invest a ton of hours in trailside side adjustments while riding the same section of trail over and over. Try front shock adjustment first. If it's to soft you will nose into the corners and make the steering heavy. Let the limiter out to far and you will just plow through the corners. The steering can get to light. A well setup sled is worth the effort. 

For the trails in southern Ontario a 600/650 is fine. No one is going to ride away from it. Even on the railbeds there are frequent stop signs and corner to corner trails wont be an issue.  If your keeping the Ace you can always run it on bag trips up north.  I ride southern Ontario exclusively these days and thats the reason I went back to two stroke. Alot less fatigue. I used to find myself looking for a motel in the middle of the night on a 4 stroke. Makes for a long ride home if your arms are beat. 

They have a few more too if you check website 

Northgate in Gravenhurst which is my dealer is being very aggressive on their non current inventory