Someone deployed a webpart to our site that left and that webpart changed the globalbreadcrumb banner of the sitecollection. The webpart is needed to be used, but I want to remove the code that is overriding the banner. I was trying to see how to do this without the actual solution that was created with visual studio. All I have is access to the deployed webpart file.
It's easiest if the webpart was deployed with a solution package. In that case, the wsp file will have everything you need. If there was no solution package, it will be harder to do, but you can still get all of the necessary pieces from the web server.
Basically, you want to recreate in Visual Studio what the other developer had. You want to use the same solution and feature IDs, so that when you upgrade or redeploy, the existing site will begin using your modifications.
Recreating the xml and ascx files is not that difficult. The trick is the code. For that, use Reflector or dotPeek to decompile the DLL back into code. You can then copy the code from those utilities into class files into your Visual Studio project.
If it is a custom webpart (probably it is, you mention Visual Studio) you are out of luck. It is a all-or-nothing deal. You cannot remove parts of a webpart, unless the developer who created it has added options for that (use the 'modify (shared) webpart' option in SharePoint to see the configuration options of the webpart).
The webpart is rendered by the code associated to the webpart (look in the webpart definition file to find out which dll). The code in this dll is responsible for all of the actions the webpart takes.
Your best option is to get the code, comment out the bits of code you don't need, compile and redeploy.
You may have a backdoor: if it is a 'visual webpart' you can edit the control it loads, although not recommended since the controls lives in the 14-hive on the server.