Well sir, there are a couple of ways to go about this, neither of which is simple. The Simplest of the methods is to SAVE AS RECTANGULAR PNG/BMP option and then trace over top. The nice aspect of this, is you can make you rivers more meandering, the coastline less smooth etc. This is much like how our World Maps get more detail the smaller the depicted area gets.

The more difficult method involves saving the whole work as a Second map (i.e. MAP-2 or some other naming convention), deleting everything that would fall outside the new border, using the SPLIT function on your coastlines, redrawing and connecting for the border areas, and then using the MULTIPOLY tool to make into a new polygon.