TOS is very buggy, esepcially the older versions. It would be wasted effort.
There are better OSes knowadays.
(And I do not refer to any MS product with that statement!)
TOS is good for using it as a the OS for an emulator- to enjoy the good old times.
The TOS was developed by Digital Research (IIRC). Perhaps the sources do not exist anymore....
However you could try to dissassemble the TOS yourself.