I rode the Amateur day at the San Diego Supercross in 2003. They had removed the big triples and cut down the whoops. Even so, it was a difficult track to ride, and there were several sections where I lost MASSIVE amounts of time because I simply couldn't do the rhythms. I was a 125 Intermediate/Vet Intermediate at the time.
It looks like the receipt you have reads "Full size Supercross track." I don't know if it will be modified for safety the way San Diego was in '03, but here's what to expect if it is. If not, expect it to be even harder:
The start will be very short. Get your starts on point! Consider 1st gear starts if it's dirt. If they use the metal grate I can't advise you because I've never started from one. Just be ready to leap out of the gate and move over on people. (Or be prepared to get moved over on.)
Use outside lines when you can. The rhythms often require speed to navigate, and most times you cannot make the first jump from the inside line. You'll be rolling the whole lane after that.
There may be some things you can't clear even from the outside. The year I rode, there was a short chicane-style section with a 180-degree left followed immediately by a steep single/table/single combo, then an immediate 180 right and the same single/table/single combo, followed by a 90 degree left. The fast line was to hop on and off the tables, but I saw 100+ people clip the second single and go over the bars into the berm. Lots of injuries that way. I just couldn't see me doing it clean every lap, so I rolled it.
Slow.
Gear your bike for acceleration. You won't need much top speed. I had outdoor gearing on and I could barely use 3rd gear.
Good luck!