I presume that is the one with the Hitec 72mhz transmitter and NiMh batteries. Has a speed 400 brushed motor.
Battery voltages would depend on how many cells in the pack.
The TX pack is probably 8 cell so up to about 11.3V for that pack, fully charged, is fine.
The motor pack that came with that RTF was a 6 cell as I recall so pack voltage is nominally 7.2V or something under 8 at full charge. So 9.6V for the motor pack sounds way too high. It may be over charged and damaged.
How are you charging the packs? What charger and at what rate?
Most TX packs want to be charged at something less than 1/2C
NiMh motor packs typically about 1.5C, but if it has been sitting for years I would bring it back to life VERY slowly, say 1/10C.
But batteries do degrade over time and if your NiMh have been sitting for years maybe they need to be replaced. The motor battery is likely to be more of an issue as it has to put out high amperage and may not be up to the task.
www.cheapbatterypacks.com can make up any pack you like. For the Easy Star I would use a 7 cell 2/3 A pack of 1200 to 1500 mah capacity if your battery charger can handle 7 cell.
Personaly I would switch to 2 cell lipo today rather than 6 cell Nimh. Lighter and more capacity. But you would need a Lipo charger.
Change the connector on the ESC to match the battery pack. cutting and soldering Lipos can be a bit tricky if you are not good with a soldering iron.