To my surprise, I found that the Power 120 is based on a PowerPC 601+ processor rated at 100MHz. It turns out that the 601+'s, part number PPC601v5-FC-100-2, from IBM are tested and guaranteed to run at 100MHz with an internal core voltage (Vddint) of 2.5 volts and an I/O voltage (VddI/O) of 5.0 volts with a max core temperature of 85°C, or 120MHz with an internal core voltage (Vddint) of 2.7 volts and I/O voltage (VddI/O) of 5.0 volts with a max core temperature of 50°C. These parts are identical to those in the 120MHz Apple Workgroup Server 9150/120. A single stack peltier device is used to maintain the low die temperature of 50°C. Unlike the early Power 100's that use a clock generator, the Power 120 has a 40MHz crystal oscillator. The motherboard has both pads for surface mount oscillators and holes for a half size metal can. The one I worked on had a half size metal can. To clock chip it, you can follow modification 1 or 2 as outlined in the Mac Crystal Oscillator Speedup History 2.5 File. Unfortunately I was only able to clock chip it to 122.1696MHz. The machine crashed at 123MHz and above. For whatever that's worth, I was also able to slow it down. I believe this is simply because the PPC 601+'s are already maxed out at 120MHz. I've only worked on this one machine, so please drop me a line at schrier@socs.berkeley.edu if you try one.
If you are interested in running Sound Manager 3.1, take a look at the Sound Manager 3.1 Incompatibilities page.