If it started life out as a 6 speed, then the spacing is 126mm. If this is the case, then you can almost certainly take any 7,8, or 9 speed hub and put it in with a slight stretching of the rear drop outs with finger pressure only. If the bike is really old as in 70s vintage (with white letter and no background panels), then it may have started life as a 5 speed with 120mm spacing. If this is the case, you may consider having a good (repeat, good) shop cold set the rear drop outs to 130mm spacing.

If the bike was set up for caliper brakes you can almost certainly convert to 700c wheels with only a minor brake pad adjustment. If the bike is set for cantis, you need to try a 700c wheel first as cantis typically don't deal with hieght adjustments nearly as well as calipers.

Bottom line, most of the old Treks can be converted to modern wheels and modern drive trains no problem.