As per the user manual for your engine located here, the battery is located in the tender and a 9 volt style battery.
BCRs work well, but I usually like to just replace the battery in my older PS2 5 volt engines. Batteries for these are relatively inexpensive, I get them from my local hardware store or Menards. Just make sure the battery is rated for 8.4 volts and at least 180mA.