The power button board is literally the exact same as the one used in the Toaster (original NES), so if that's all you're wanting to do, you don't need any special guides--just look up the details of how folks did it on those consoles. Technically speaking, you'd just need to figure out which leg is which on the LED, desolder the original LED, then solder in whichever new one you want. I did literally the same thing this weekend on a couple of Apple IIgs boards a guy I know wanted updated with color changing LEDs.
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Need Stadium Events + Panesian titles, 6-in-1's, Menace Beach, Moon Ranger & Secret Scout to complete my licensed/unlicensed sets.
If you've got any of the above for sale, hit me up!