Hi! I can't find an instruction manual with the controls anywhere, and have no idea how to accelerate! I am able to boost and steer, but that's all. I'm using 2-button Immortal Joystick.

Check the pdf included in the archive for all details.. But it is:

RACE CONTROLS

Once the stage is running, the racing controls take over.
Accelerate Up, Fire Button 2 or CD32 Blue Button
Brake Down
Steer Left / Right
Turbo Boost Fire Button 1 or CD32 Red Button
Pause P key or CD32 Play/Pause Button
Abandon the run ESC key or CD32 Reverse Button