If the old tarp is urethane coated nylon, which it probably is, you'll have eactly the same problem with deteriorating coating.

Cheap muslin (bleached or unbleached, doesn't matter) runs $2 per yard at JoAnn's and you can usually find a 40% off coupon. You shouldn't need more than 2 yards to make a mockup of your pack to test for fit.

A small remnant of ripstop nylon (just enough to test your machine) shouldn't cost more than $2. You only need a small piece, enough to test stitch on your machine.

Again, with the amount of work you put into a project, it's worth getting good fabric so that your many hours of work aren't down the tubes in a year or two. It's still a lot cheaper than buying a new pack.