Phototransistors seem to be generally unavailable now

[IcedQuinn]

Seem to be sold out (Digikey) or marked as won't be stocked (Mouser.)

I put a pin in looking up suitable replacements but I have to figure out what the rules for part substitution are here (i'm not a hardwareblob.)

[JesusFreke]

Oh, interesting. It looks like LCSC has the related PT908-7C-F, which I believe is the same thing, but just with a clear package instead of the black package. I'm not sure if that would affect anything, but I suspect not, since the PT will be basically entirely enclosed by opaque plastic.

https://lcsc.com/product-detail/Phototransistors_Everlight-Elec-PT908-7C-F_C142281.html

Someone else who is building one made a modified design that uses surface mount components instead, so that may be an option if these become unobtanium.

[JesusFreke]

liteon LTR-301 looks to be the same form factor and wavelength. although it looks like it might have a bit of a lower sensitivity, so some of the resistor values might need to be tweaked.