American Luthier magazine had a very good article on sitar building a few years back. Cant remember what issue number and it didnt offer any plans but a very good article non the less. I've searched for sitar plans in the past but had no luck at finding anything.
What kind of information are you looking for, I built my own mayuri, I may be able to help. What you need to do is get a "Shitar" and strip it apart and use that as a pattern. Materials are easy to find here with the acception of tumba, I grow them, but for stock on my own instruments, however if contact a maker in Miraj, they can help you out.
Beenkar Ted Ceplina
I've seen this question many times, but I think the best answer is indeed to "reverse-engineer" one of those "shitars". You can find them on Ebay for well under $100 to use as a pattern! I have grown gourds before. It is a long, painstaking process and only one out of maybe 100 is suitable for working into a resonating body. But at least nature does the hard work! You just provide the dirt, water, etc.