You can take Corridor 15 (Rail Bed) from the Weirs and follow it toMoultonboro.
It is basically all by land.
If you wanted to try by Lake get over to Moultonboro Bay and when you get up to where Corridor 15 crossed the Lake to the Castle go left and back up 15 to Local RT 201.
Good Luck, Hopefully the snow will still be good by the weekend. I am not sure if there is a "Club" ride this weekend?
I have been asked where is the Map I occasionally use to show the Route of our Trial Rides. Below is My secret source.
NHSA Interactive Trail Map
http://www.jimapco.com/maproom/snowmobile/nh
You can adjust the size of the map to add or reduce the area you are looking for.