If you have an existing model I would convert the SBML into Antimony format and then edit that. Antimony will give you a list of reactions, species,rate laws etc, Any changes you make can be converted back to SBML, or loaded straight into the tellurium simulator.
For example, if you have an SBML file or a string containing some SBML you can do:
import tellurium as te
r = te.loadSBMLModel (string or file name of SBML model)
To get the Antimony you can use
antStr = r.getAntimony ()
You can print out the antStr and copy the string to a new editor window.
What I generally do is if you're using the Windows spyder distribution of tellurium, there is an import sbml option in the file menu and that will convert the SBML to antimony and drop it in the editor windows.