Hello Groovy !
Lots of luck trying to put a sound board in a Doodlebug . first off I don't have a clue want year mine is ? what they call the power truck , is powered with a shorty drive shaft from the motor to pull the Bug . I took nine apart and found out the gears on the axel shaft have split and can no longer pull the Doodlebug . I hope this answers you question ?

Hello out there .  I just talked to the Bachmann parts Dept. looking for the part number H81409  power truck drive for my Doodlebug , only to hear item is discontinued and no type of replacement parts available . Anyone know where I can find parts to fix this little bug ?   

Bill