I've picked up a couple of these units (#21357 and #47716)(both say model 98 but are not identical) and hope to get both back to usable condition. The antique radio restorers generally start by renewing the power cord, and "re-capping", which is replacing all of the electrolytic, paper, and molded paper capacitors as a matter of course, don't even bother testing them first (mica and ceramic capacitors, by contrast, tend to be very reliable). And of course, blow out the dust, clean all contacts, and make a visual inspection before ever plugging in and turning on the unit for troubleshooting. I'll do this, but if one of you happens to have a wiring diagram or schematic that you'd be willing to photocopy for me, please send me a PM.

(EDIT) FWIW, the February 1958 issue of Boat Sport magazine ran a several page article on the then new Merc-O-Tronic. The collection of Boat Sport and Speed and Spray issues from the Fifties is fun reading.

http://boatsport.org/BSV65/p26.jpg