I took the plug out and carried it to several auto parts stores and only find two sizes that come close.
One o-ring fit really well, but wasn't thick enough to seal well (OD measurement is 7/8ths). I test fitted it and it slide right in, but the plug was loose in the transmission case.
The other o-ring was thick enough, but was very loose fitting (OD Measurement is 1"). I could freely spin the o-ring around the plug and when I test fitted the plug it into the transmission case, it prevented the plug from locking into place.
Thanks