I like this route as an extended day ride. Probably not suitable for street bikes so much simply because a lot of distance is covered in a day. As a day ride beginning in Canada factor in the border crossing south as 1 hour. Travel via Hwy 5 to Sedro Wooley and begin The North Cascades Scenic Highway to Twisp. The view at Ross Dam is beautiful and Winthrop makes a great halfway spot of the total journey for lunch or coffee and a stretch. Truth be told if you roll up to Twisp there´s an independent gas station that makes great knock off sub sandwiches for cheap and gas is also tad cheaper in Twisp.
As you motor on to Oroville and cross back into Canada at Osoyoos and begin south on Hwy 97/3, the scenery is very nice and you forget that 400+ kilometers is in your rear view.
In my opinion the best window for this ride is late July and August because the weather in North Cascades National Park can be unpredictable in spring and fall.
Tip. Always bring rain gear for this trip and stay hydrated.