Welcome, Matt38. Typically your JD 25a has one piece cup knives or hammers. If so, you would not be able to install dethatcher blades. A normal dethatcher setup has 2 piece "Y" blades and a dethatcher blades is hung between the Y.
You could easily make your own dethatching blades with some flat 1" to 11/2" wide steel, cut to a length slightly longer than your hammers and drill the appropriate hole and install.
Figured I might have to. Wasn’t sure of the importance of the heat treated steel but if I had to I could order some and cut to fit!View attachment 568833
No, you cannot easily make dethatching blades. John Deere does not offer a dethatching knife for the 25A flail mower.
If your intent on doing this rather than rent a walk behind dethatcher from big golf course mower dealer you should call flail master and ask if they would send you a couple of the Tiger TO1T dethatching blades as you need to make sure that they will clear the flail mower shroud when it spins and not damage the mower.
It may be simpler for you to spread gypsum to dissolve the clay and pelleted lime to choke out the weeds then spread new seed on the field after that and be ready for the fall and winter.