You'll need the developer tools (Xcode) installed, unless you want to rummage around in VersionTracker or whatever to find something that can change classic type/creator codes.
First, go into System Folder, control-click on Finder, and Duplicate.
Now you have a classic Finder that can be run as an application. You'll need to do a File->Open to get a window going because the classic desktop will be hidden.
The Classic Finder doesn't have a Quit item in its menu, so you'll need to shut down Classic manually to make it go away.