Yes, by configuring a CPU limit on your VM, you can confine it to a specific set of clock cycles.
It's important to note that the guest will not see its CPU as being 66 MHz, only that it will be artificially throttled to this rate by the hypervisor.
EDIT: I just saw you posted in the Workstation forum. If not obvious, the above screenshot is from vCenter which may not help you at all.
You could add the following lines to the vmx configuration file.
cpuid.1.eax = "0000:0000:0000:0000:0000:0100:0011:0011"
timetracker.apparentHz = "66000000"
The cpuid.1.eax value of 0x00000433 just makes the CPUID return an Intel 80486 while the second line makes it appear to have a 66MHz clock to the VM.
Note that it is just the CPUID that is changed; not the capabilities/features. If you want specific features masked out to be similar and/or exactly same as an 80486 it will be more complicated than that.
The timetracker.apparentHz value won't change how fast the instructions are executed in the host CPU; so it won't make the VM run slower.
you cannot do this in VMware Workstation.
But do note that detail... while you cannot do this in VMware Workstation, you can actually do something very close to this in VMware ESX/ESXi
You an set limits on the CPU speed in the resources tab of your VMs configuration and down there you can set for example a limit on 100MHz.
No. ESXi is a bare-metal hypervisor, whereas Workstation/Player/Fustion (for Macs) is a OS-based hypervisor. The bare metal one has full control of the CPU, and therefore can adjust the resources appropriately. The hosted ones must play by the rules of the host operating system.
The best you can do is use a slowdown utility inside the guest VM.