SOLUTION

Converting from MKS to Perforce requires first converting from MKS to CVS, and then converting from CVS to Perforce. There is no known, public script to convert an MKS Source Integrity server directly to Perforce. 

There is an MKS to CVS conversion utility available on a SourceForge project site. The primary documentation for the MKS to CVS converter is the file "si_to_cvs.txt".
 
Once you have successfully converted from MKS to CVS, you can then convert from CVS to Perforce using the CVS2P4 utility. The CVS2P4 utility is a set of Perl scripts that are freely available in the Perforce Public Depot. More information on the CVS2P4 utility and the CVS conversion process can be found in the related Perforce Knowledge Base article CVS to Perforce.

Neither the MKS to CVS converter nor the CVS to Perforce converter were created by or are officially supported by Perforce. Nonetheless, Perforce Support has expertise in conversion issues and is available to assist in the conversion process where practicable.

Notes

It is possible that the MKS to CVS converter could generate a CVS repository that contains branch names with disallowed CVS characters. It is also possible that any CVS repository can admit additional keywords that disrupt the parsing of the CVS2P4 converter. The presence of disallowed characters and RCS keywords is relatively rare. However, if the CVS2P4 converter fails to parse RCS-style archive files originating from MKS or CVS, then the original RCS files must be examined and the problem characters or keywords expunged prior to using CVS2P4. Please see CVS2P4 Troubleshooting for more details about what keywords are disallowed by the Perforce Server RCS parser.