You are going to have to give more information. Do you mean illegal in the P-P Class? If you mean P-P, and your engine was originally 883cc and is now 1000cc, then yes, you could not run in the PP Class but you could run in another class such as the 1000cc (if it is under 1000 cc,) or 1300 cc if it is over 1000cc. There are lots of classes you can still be "legal" in. Just be creative and match your bike to the class you want to compete in.
Not to denigrate the HD, but it could never compete with the 4 cylinder 1000cc bikes like Kawasaki and Suzuki.
Don