Hi Jack, I've ridden this area for a few years. On the east side of the lake there is trail 105a you take that about 4-5 miles and it will lead you to the trail to go across the border. It should be well marked. Crossing there can be a little confusing if there is fresh snow and you can't follow other tracks. If you care to, you can stay on 105a east to Norton, and cross there this is the easist way to cross but about 25 miles (usually nice trails)
If you are staying on the west side of the lake, you can cross the lake in town, or to the north which hits trail 105a . I personally stay away from Maggog and Sherbrook as they are busy industrial cities where the trail goes through and some very busy road crossings not exactly nice scenic countryside. Riding east to Coatacook is nicer. Happy trails, the dude