Welcome to the forum. A Google search took me to Sears, it might be hard to find an original. If you can't try Googling: thin 13/16 wrench
Example: Amazon

 

Hello and welcome to the router forum.
Sommerfeld tools sells them
Bent Wrench 13/16

 

welcome to the forum, Lewis.
are you a DIY kinda guy that likes to do things outside of the box ?
I made a collet wrench for a hand-held Dewalt out of a lawn mower blade quicker and
cheaper than I could buy one. all you need is an angle grinder and a metal cut-off blade.
(and of course a flat piece of steel)
or - if you can find the common thin 3/4" flat wrench, you can grind it out to fit.

 

Welcome to the forum.

 

Look for tappet wrenches. Or cone wrenches

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Welcome to the forum. Not having seen the wrench any 13/16" wrench should work but the main question is if you need an upper/lower wrench set or does the router have a lock pin requiring only one wrench. Many thin wrenches to choose from and others are offset if that's needed. If it's only the one wrench then neither thin or offset is likely needed and a regular open end wrench should do fine.

 

Check out MLCS or even Harbor Freight. Often times HF has more less or throw away wrenches with their products.

 

You might find a tappet wrench at an auto parts store.

 

Welcome to the forum I have made my own several times like John recommended just draw out the shape you want and cut it with an angle grinder. I have an oxyacetylene torch I cut mine out with that then heat treated it. If you are not into the metalworking stuff or don't have the tools here is one at MSC extra thin wrench 13/16 on one end and 7/8 on the other end MSC wrench