Is the shifter just kind of flopping around, or does it feel like it's locking into gear?
If it's flopping, it might be just a pivot, cable, or a linkage - a relatively easy fix.
If it's engaging, then you might be looking at some major repairs.
You have a loose connection either on your power (red) wire, or your ground. When you hit a bounce or whatever the connection is being broken and restored.
You probably would want to start out with something a lot smaller than Ninjas and GSX-R's.
I'd look for something in the 100-300 c.c. range to start. You can move up to something bigger once you get comfortable.
Don't knock the little bikes until you try them.
It can be a few different things.
First thing that I'd check is the fuel lines underneath the car. Are there any drips?
The next thing I'd check is under the hood. Any leaks there? Just follow your nose to find it.
If you can't find any leaks or drips, you might have a hole in the top of...