ghunter said:In my youth - MANY years ago - I rode an '86 YZ125 at Tower City most weekends. The bike was fine as long as I jetted it properly for the elevation and temps I was riding in and kept it cleared out regularly with some wide open riding. If my memory of 20 years ago serves, there are enough open areas to keep a 125 opened up and have fun on the trails, too.
As for the rocks, install heavy duty tubes and run your tires closer to 16-18psi instead of 12-14psi and you should be ok. The extra pressure will help prevent the rocks from cutting into your sidewall and puncturing your tubes. Carrying a spare tube in the size for your front tire and some tire changing tools is also a good idea, just in case.
Have fun and good luck.