Hey Jeff, if the 8989 uses the same cassette mechanism as the 777 or the 9494, I'd say you need the following:
SBM6.0 (Square Belt Medium)
6.0 Inch / 15.2cm Inside Circumference
.051 Inch / 1.3mm Thick and Wide
FBS11.5 (Flat Belt Small)
11.5 Inch / 29.2cm Inside Circumference (total length)
.025 Inch / .6mm Thick
.125 Inch / 3mm Wide
On the GF-777, the following belt is for the tape counter:
SBS7.4 (Square Belt Small)
7.4 Inch / 18.8cm Inside Circumference (total length)
.046 Inch / 1.2mm Thick and Wide
Maybe jaetee or Uncle Ed can confirm this.