So that's how they work. Thanks for the explanation.

You could make a 3d model of it and print it. Maybe I will have a play with it later this year.
I will need some dimensions of the sensor for that: housing and magnet sizes. Are the magnets very strong?

You can try to use https://www.3dhubs.com if you have a design.

Alternative to silicone fill a shell, is to use hot melt glue to fix the magnets. Just be carefull not to overheat the magnets.

In 3d printing complexity is free. You can think of complex shapes and not a simple shell. I would make a housing where the reed contact and the magnets would slot in. A small drop of super glue to fix it all.

What distance can these sensors still detect a ball?

I did some experimenting with parts laying around. Using two 10x2mm round magnets and a 15mm reed contact, a ball could be detected 9mm away. Impressive. With two 3x2mm round magnets the sensing distance was about 3mm. Need to order some more more size magnets like 5mm and 8mm magnets to test.
Drawback of these sensors is that the ball is attracted by a magnet. Not much, but still noticeable. DO the original sensors also do this?

I tested with neodym ones (unknown strength rating). These where the only ones I had laying around.

Near the maximal sensing distance the ball is not attracted very much. Moving the ball over the sensor you can just feel it. I think on an angled playfield the ball will just roll of it.

I would not think using different magnet material would make much of a difference.
The pinball need to capture enough of the magnetic field for the reed to react. So it would always be attracted to the magnet. The magnet strength would likely be the main driver for sensing distance. That sets the field strength around it.
Possible differed reed contacts would improve the sensitivity, and thus decreasing the change in field strength needed.

Nice design. It looks like an injection molded part. Hope you can find the correct size and strength magnets for it.

I just published on thingiverse a 3d printable sensor pinball sensor with instructions how to build one. See here: http://www.thingiverse.com/thing:1652237. The maximal distance to detect a pinball is about 4.5mm.

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My switches connect directly to a digital input so I don't need a diode, but If you use them in a switch matrix then you will need to add the diode.