That would be challenging. Its not a matter of replacing a quartz oscillator like in the eighties. HDDs run very integrated SOCs. Actual SATA, DMA state machines, everything is clocked from same crystal so you would have to know internal architecture very well (clock tree) and know actual internals of the PLL driving everything (registers/datasheet) to be able to reprogram/temporarily slow down just the CPU speed while maintaining SATA link.