Hi,
There is lots of 3rd party tool. But anyway Sharepoint designer is good tool for migrating web part.
For more details look into this;
1. How to upgrade custom SharePoint Portal Server 2003 areas to Microsoft Office SharePoint Server 2007 (
http://support.microsoft.com/kb/555858)
2. How to Upgrade an Area based on a Custom Site Definition (
http://blogs.msdn.com/sharepoint/archive/2007/03/06/how-to-upgrade-an-area-based-on-a-custom-site-definition.aspx)
3. Plan Web pages (
http://technet.microsoft.com/en-us/library/cc263106.aspx)
Happy Holidays