you begin by saying what expirence you have in the following areas:
motorcycle frame modification
working on bikes in general
also what kind of equipment do you have access to?
Tony Foale has a book out on motorcycle frame design but it is tres expensive (used copies are cheaper on amazon).
My advice however is that unless you can lay a structural weld, and you know what rake and trail are and how to calculate them, and you have access to a mandrel tubing bender, and you have lots of cash, you are better off buying one of the many featherbed replicas out there and modifying it to fit your own engine.