...it is a very straight forward and easy-going task to replace the seal. Too many posts already on that subject....
Anyway, there are plenty of places and people that will help you in the Chiang Mai area. They will help as best as possible to keep you going.
David Unkovich was still there in 2008. Maybe he is still around. He definitely knows the area. There is the ex-Pat bar that has plenty of riders there nightly. It has coordinates:
I don't have the addy for it. There are plenty of rider and overlander bars on that road. There is also an old skool bobber shop located between the river and the main road in town. Ask around and riders will direct you to their shop.
Enjoy your trip.