You might see a clockmaker or repairman for the square head wrench-it doesn't have to be very strong and most wind-up clocks use a square head key.
The filters should be fine if the sextant has been kept in it's case. They're really not out in the sun that much that they should have faded. Be aware that not all the filters are dark. The horizon shades have to deal with fog and glare which call for just a slight shading.
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.