Hi,
From the error message, we can see that you're missing a reference to the assembly that contains WebPart classes.
Please Add Reference to the Microsoft.SharePoint.dll assembly so that you project can be built successfully.
Also, in ASP.NET 1.1, you will need to use Microsoft.SharePoint.WebPartPages.WebPart class to create a webpart.
HTH. If this does not answer your question, please feel free to mark the post as Not Answered and reply. Thank you!