LED emitters could likely be reproduced fairly easily, however they do pop up cheap on occasion (~$10 a board, or ~$100 for a full chain of 10) which would likely be much less than you could reproduce them for.
The gun protect board would be easy to build yourself and indeed it started as a small board that a Sega Tech was building by hand.
I don't foresee anyone reproducing the sensor modules though, the sensor itself is a bespoke part that you can't buy alone, and it requires the plastic housing to protect it and keep it from getting misaligned. that needs to be an injection molded part in order to ensure the IR light can travel through it clearly. That alone pretty much kills most hopes of getting it reproduced.
The main gun board could potentially be reproduced so long as it's not using any custom chips (I'm not sure if it is or not), but like the LED boards they often sell in the $80-$100 range so it's not likely that a reproduction board would be cheaper.
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When it comes down to it the sensors are the most expensive part, and they're the most expensive because they're using custom parts which are not easily reproduced.
I'm pretty sure all of these parts can still be purchased new from Sega which also brings in the legality of making reproductions.