Is your Series One CPU compatible with DirectSoft32? If so, then you could import your TISoft program file into DirectSoft32 and download the DirectSoft program file to the PLC. Even if you don't already have DirectSoft32, it still might be easier than finding an old copy of TISoft, not to mention finding a PC that will run it and communicate with the PLC.
I keep an old GE Fanuc Workmaster II 386 computer with DOS just for jobs like that.