I'm sure some of you have seen this already: Ronald Walters published a great list of videos on youtube on how to build a small portable mechanical organ. This is not quite the same as a full pipe organ, but a lot of things such as pipe construction and bellows are similar. It is played by a paper roll and not by a keyboard.
Here is the playlist:
John Smith Senior 20 Organ Build
https://www.youtube.com/playlist?list=PLDjucfhdTRPlQbnNCd848nNnnwNdTnk5RAnd an example of the finished product:
https://www.youtube.com/watch?v=dHSErIHZUKE