As I suggested if you want to learn about the changes, get a copy of the book I suggested. There are pages and pages of information published over the years and would take hours to go over all the nuances of what makes a 1913 and are different then a 1912 or 1914 for example. People have spent a good part of a lifetime learning this stuff and new information is still coming to light.
Lang's sells a lot of the sheet metal and other add on parts to build a speedster.
https://www.modeltford.com/item/NO.800F.aspx
https://www.modeltford.com/item/3060DE.aspx
https://www.modeltford.com/model-t-p...ipment/?pp=100
https://www.modeltford.com/item/3923LV.aspx
Suggest asking specific questions such as your engine question. Another good source that might be of help is the Speedster Facebook page.
https://www.facebook.com/groups/modeltspeedster/
Add one bit of information, the rearend is 1926/27 big drum along with the reproduction AC/Rocky Mtn brakes.