The Williams 20-10293 Reed Switch is nearly unobtainium and used in several Williams/Bally pins:

Cirqus Voltaire
Cactus Canyon
NBA Fastbreak
No Good Gofers
Revenge From Mars
Safe Cracker
Star Wars Episode 1

And if you can find them, they cost $20 -- pretty expensive for a switch. I had a dead one in my NGG, and pretty much gave up replacing it as it wasn't essential for game play (one in a set of two in front of Buzz for detecting hits). However, after reading this post by german-pinball:

https://pinside.com/pinball/forum/topic/has-anyone-found-a-replacement-for-the-20-10293-reed-switch#post-2441237

I thought I'd give it a try. And it worked. And it was pretty easy. And it was cheap ($1.50 for the actual reed switch). So to expand on german-pinball's short instructions, here are step-by-step instructions with pictures.

Parts:
Magnetic / Reed Switches 1 Form A 7mm AT 1015: http://www.mouser.com/ProductDetail/MEDER-electronic-Standex/KSK-1A80-1015/?qs=KFo7JewZbUFX%252bU5EsD5Ziw%3D%3D
Diode: http://www.pinballlife.com/index.php?p=product&id=173
Silicone or caulk

Tools:
Soldering gun and solder
Helping hands (they really help)
Small flathead screwdrivers to pick and pry
Wire snippers

Step 1: Take out reed switch
Step 2: Pick out silicone (whatever it is). I used a small flathead screwdriver.
Step 3: Expose diode and reed switch

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Step 4: Pry out magnet
Note: Don't pry out magnet on the other side of the reed switch.

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Step 5: push out reed switch and diode with switch wires.
Note that there are small channels the wires run through in an internal bracket and then between the magnets

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Step 6: Remove old reed switch and diode (might as well replace everything)
Step 7: Tin new reed switch and place in channel
Note: orientation of reed switch *may* be important (need to test). In this example, both the original and the replacement were oriented with the flat sides of the switch facing up/down (not looking at the edge from above).

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Step 8: Tin new diode and place in channel. Note the direction of the diode!
Note: You'll have to cut the lead to fit

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Step 9: Solder leads of diode and reed switch together
Note: just a quick tap with a soldering iron should do it

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Step 10: Solder plug wire to reed lead. Clip excess lead
Step 11: Solder plug wire to diode lead. Clip excess lead

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Step 12: replace magnet

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Step 13: Fill with silicone or caulk. I had a tube of 100% silicone for aquariums on hand. Let cure till surface is solid.

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Step 14: Put back in game and test with switch test.
Step 15: Play some pinball!

Thanks GPE! Do you sell the reed switch? I'd like add a link to it, if you do.

Thanks GP! Wouldn't have been able to do this without your work. I'll send the link to Chris to see if he'd be interested in it for his wiki and I agree that it would be great if a small kit was available through GPE.

I may have had some dumb luck, but does it matter if the switch is rotated 90 degrees so that, from above, you are looking at the sides of the leaves in the switch (as asked by GPE)?

Neat tool! I have a couple of extra reed switches from Mouser. If anyone wants to send me a broken switch that I can test, I'll do my best to fix it and send it back to you. Just PM me.

I updated Step 7 to address the orientation of the switch (just in case).

Based on what German-Pinball has said, I think length is the big factor in these switches. Can you open it up and measure? If you want to send it to me, I'm happy to try one of the Mouser switches and test it out. I'll send it back to you regardless.

Nice. This should help people understand the orientation.

I requested a new account to do this, but never was authorized. Anyone else have one that can do this?

Very nice! Thank you GPE. Are you carrying The reed switch?

Fantastic!

Yes -- and that also explains way the orientation of the reed switch is important. If put in 90 degrees offset from how it is supposed, the lower field won't push it closed.

Awesome work, German-Pinball. Most places seem to be out of stock, but here is one for $20: http://www.flippers.com/catalog/product_info.php/reed-switch-p-4953

It would be great if they can be remade.

I have never noticed any grab by the sensors on a passing ball on an installed switch, which I've done many, many times while testing these switches. However, I haven't tried with the sensor out of the game. Most of these are mounted below plastic (ramp, insert), so 9mm range is probably too much. In any case, try testing through a ramp and see how much the magnets in the switch grab a ball.

Nice!

Yes -- that is my thought as well: 3-5mm sensing should be perfect.

Nice work! This is exactly the missing piece that people need to build their own from scratch.