They are probably more similar to Teleflex cables used for steering & control applications for boats. I know you can get stuff for marine applications that may have a cross over in size and strength to the old power top drive cables.
If Hydro-E-Lectric can't help, you might call this outfit.
http://www.convertibleparts.com/index.htm
They make drive cables for the 71 to 76 GM products that used similar drive set ups. The key would be a company that has the right size cable and a way to swage the drive ends to a shape or fitting that will work for the Mercs that used them for a time.