The simplest answer: DONâ€T! Take the window to a studio and get some advice. The proper cleaning and restoration of these beauties usually involves properly handling and disposing of some rather hazardous materials and waste.
a) I am constantly asked questions concerning lead and how to handle it. In general, you want to limit your exposure and keep it as low as possible. Volumes have been written on this matter with all kinds of technical language. There are a few simple rules: THINK, CLEANLINESS, proper disposal of waste, never ever grind or sand anything containing lead including old paint, never breathe lead-bearing flumes, and never use antique lead eating utensils.
b) For lead paint, these two links are informative - copy and paste.
A Safe Way to Remove Lead Paint
Lead-Based Paint Regulations