Welcome to the forum!
In addition to tripledaddy's comments, you could also check your local salvage yard. The cap for your car will likely be very similar to many other Ford products, so you shouldn't have a problem finding one.
As for next step after you get a replacement master cylinder cap, I'd HIGHLY recommend you flush the entire brake system with fresh fluid. Brake fluid is hygroscopic, so being exposed to the elements pretty much guarantees that you'll have a lot of moisture contamination in the fluid. That moisture, in addition to reducing braking efficiency, will also potentially cause corrosion in the ABS pump and may render that ABS pump faulty. The ABS pump is not inexpensive to replace.
-Rod