On the mini-bike. Your choice - put the new engine in and your power more than doubles. But you then lose it for building something else.
If you want something simple to wake it up - then put in the smallest drive sprocket possible. I didn't quite get your explanation. How many teeth are on it now? Sounds like more than 12. Get a #35 with as few teeth as possible. If that doesn't work, then the bigger wheel sprocket is next up...