What made it possible to track down the RNG function was that the Computer History Museum has a big archive of Alto code [1]. By searching that site with Google, I found the random number source code. Of course figuring out the code is a bit of a challenge since it's in BCPL or Nova assembly.

Hopefully as part of our Alto restoration we'll be able to add more old Alto disks to the online archive. Maybe the disk labeled "Alan Demo" will have an exciting long-lost Alan Kay demo on it :-)

[1] http://xeroxalto.computerhistory.org