Many of the trails above Leavenworth ARE pretty technical. There is one trail loop up by Lake Wenatchee (west of Leavenworth) called Chiwawa (I think) that is more intermediate. It might tie into the Mad River system as well. I think you have to go back through Wenatchee to get to the Mad River trailheads. They are north of Wenatchee off the Columbia River.
There are some trails to the east and west of Highway 97, south of Leavenworth. I've been on Tronson Ridge trail on the east side of the highway. It's fun but no real loops to be had.
These trail systems are "official" offroad vehicle trail systems. If you stay on the trails you don't need a street license, but you do need a state ORV sticker. The ORV stickers are sold by state licensing agencies and should be available by mail. Not sure how that works with a Canadian bike. You're not supposed to ride the dirt roads unless you are licensed. There are law enforcement people around the trail systems on summer weekends and they do write tickets.
In general, the area right around Leavenworth is not very dirt bike friendly. It is more of a tourist town, with hiking, mountain biking, rock climbing, whitewater rafting, etc., being popular. Also lots of pirates on loud bikes.
Check out
THIS link - select "Wenatchee/Okanagen National Forest". It's got the scoop.