Well... if you do have several (same based) engines you can (for a part) translate their behavior into the numbers. With the list on first-racing you can see some box stock and modified engines next to each other so you can learn the basic ideas of some modders about changing timings. And if you go to some kids in the neighburehood who are into modifying 2-stroke motorcycles you can learn from them.

And ofcourse not to mention the book Jennings 2-stroke tuning guide