Timothy,
Here are some links to get you started, this list is incomplete, and I will edit / add to it.
Variations on the theme by Jeff Davis:
Here is a collector, talking about what antiques you may find for sale in North America, rather than re-inventing your own.
Here is a typical Asian walk behind tractor YouTube channel with lots of interesting videos to waste time with!
Edit 1:
Here is an enterprising young man from Viet Nam who uses some impressive welding and workshop skills to build a largish garden tiller from scratch. this will provide some ideas of what will work for you, and what will not. There are three episodes, this is the first, the other two links are provided at the end of the video.