In case you're interested: I realized that I indeed already have System 3 binaries and source code in my collection; unfortunately the original links at ETH don't work anymore; but I have a physical copy of the book "The Oberon Companion" which includes a CD; I now have uploaded the CD contents to Github; here it is: https://github.com/OberonSystem3/TheOberonCompanionCD.
Note that there is no Coroutines.Mod; I neither found one in the A2 sources.