Can a CPUs speed be increased by (physically) upgrading the CPU clock?
I read somewhere [...] that the real limiting factor being electron travel speed in the circuits, which wont be met until extremely high THz rates, or well over PHz.


Pure fiction. Electron travel speed itself is relatively low. Electromagnetic wave travel speed - that is the interesting one - is in the order of the speed of light. At 1 THz - or in 1 ps (picosecond, 1e-12s) - your signal would travel just 0.3mm.


  what would prevent me from speeding up a CPU if I replace the clock


The critical path would prevent you from going above a certain frequency that is usually not much higher than specified. In a nutshell this is the signal path that takes the longest time, but must be finished in one clock cycle. Once you rise the clock speed above that limit, the CPU will no longer operate correctly.