The battery door catch is on the B parts sprue, part number 0004422. Available from
Time Tunnel,
Fusion Hobbies or can be found on ebay.
If you planning on getting some use out of it you are advised to replace the hex driveshafts with the pin drives or the universals from the Frog rerelease. Tamiya even make hex drive converters, part number 53913, that lets you fit modern 12mm hex drive wheels on the rear.
You can download the manual for the rerelease here
http://www.tamiya.com/japan/download...54the_frog.pdf