Well, for the cost of upgrading the fork, drive train and tires, I would have to suggest getting a new bike. Especially if you put it through the rigors of a Canadian salty winter. A new respectable fork will cost you around $300 Canadian, and a new drive train probably around $175. That coupled with the new tires brings you probably somewhere close to $420 for the components you were looking to upgrade.
Perhaps keep this Hahanna as the commuter and buy a new 2005 Hahanna to use on trails? If you do go this route and decide on an alternate bike, search around the forums for advice on buying a new bike as the selection has probably changed slightly since 2002.