There ARE indeed "cheat" codes. They are actually progress codes that you get for beating levels.
Here are some hints for getting a high score. Get 5 power ups. Then, hit a ramp at max speed (25mph) and go straight up in the air. In the air, pull a Drill Bit with Fred or a Wing Walker with Dave and hold them as long as you can. The tricks that you can hold give you a LOT more points. Also, do a Triple Flip while doing them. If you can, land backwards or in a Seat Spin (press X and Z at the same time) to continue your combo.
I've gotten about 200,000 on the first level, so don't give up!
Use this code in the course editor to test your tricks with:
7,1,1,6,1,1,1,1,1,1,1,1,1;0,0,0,0,0,0,0,0,0,0,0,0,0; 2:100:60,2:100:60,2:100:60,2:100:60,2:100:60