Good luck Matthew! Good news is if the Site is ASPX or ASPX+Portal, it's not extremely difficult to rework, i've converted over a couple sites already.
First if you have ASPX (Not ASX+Portal), try adding the webpart zones to your aspx files and changing the page template from ASPX to ASPX+Portal, that at least will allow you to start using webparts / widgets.
To go purely portal, the process is first to rework the master page (copy the master page's aspx layout into the "Layout" Tab of a page template, and replace each piece with webparts).
After you do that, you will just one by one go through the page templates (not pages, just the page templates) and copy the ASPX code to the Layout tab, add in webpart zones and replace the functionality.
Keep in mind if there is any logic in the back end (aspx.cs) file, you can add this to your portal page templates through server script tag in the layout, or a custom 'webpart' that does the logic in a control.
Worth the transition.