Just got me thinking that a Whippet could be upgraded to a Surf Squirrel if you could get the necessary chips and new the pin-out for the SCSI connector.
Unfortunately there aren't any photos with sufficient detail to identify the missing components. I have fired off an email to Hi-Soft on the off chance that they have the schematics filed away in a cabinet somewhere.
It does at least seem that it's possible to buy the AM53CF96KC

Surf Squirrel.

Whippet.