I've done a lot of reading through here for my pump service and other items.  I appreciate all the helpful information.  Regardless I managed to make a wrong turn.  I'm always surprised at some of the things I do after 35 years of working on mechanical stuff.

I think I may have botched the water pump job in a way no one else has before.  On my 1985 K100RT I was trying to get the last, deepest seal out of the front of the pump body and was not having much luck.  I thought I detected some space behind the metal portion of the old seal so I decided to drill a hole in it and tap it with my slide hammer like I have done with other difficult-to-remove seals.  There was no space under the seal.  Unfortunately I stupidly drilled right through to the other side of the pump body.     Looking at the first photo, in the front of the pump where the seals go you'll notice two holes where there should be one. The bottom photo shows where the hole comes through the back side of the pump.

I figure I have 3 options now. 
1.) I can pitch the whole pump body and buy a new or used pump body.
2.) Repair the whole with two part epoxy for metal, something like JB Weld (https://www.jbweld.com/collections/epoxy-putty-sticks/products/steelstik-epoxy-putty-stick).
3.) Repair the whole with dura-fix (http://durafix.com/index.html), which is like an aluminum braze.

With #3, I have to heat the aluminum to 735 degrees F and I'm concerned I may warp the sealing surfaces of the pump body. 

With #2 I'm concerned that the little patch will fail and introduce an engine damaging piece of hardened epoxy into the oil stream.

Any thoughts?  Lately I'm leaning toward the JB Weld.

It may have been a case of "don't fix it if it ain't broke".  I didn't have any coolant leak, but old, leaky oil was caked on really thickly.  I figured the mating surfaces were leaking.  I have the bike torn down for a lot of other work so I wanted to address the leak.  As it turned out, the threaded stud at the front end of the drive gear was mostly sheared but barely hanging on to secure the water pump impeller.  I know I didn't shear it while removing it because I barely turned the nut and it came off.  Plus, the the sheared metal was corroded like it had been exposed for a long time.  The last photo shows that. Has anyone encountered that before?

re the sheered bolt
yep pretty common thing .

I recon if you look around , you may find another pump body pretty cheap ,,as there have been many attempts , and new replacements sought after the failed seal replacement ..BUT  you may need to source a newer Big gear shaft , unless you can get a machine shop to drill and thread a new recess   through the front of the shaft  to accept a 6mm bolt ,(the obvious thought is that the shaft maybe case hardened and hard to do this )

whilst thinking about the shaft , how does it look ?  is it quite corroded  where the seals mate up to the shaft ?

if your lucky this oil leak (rather than water leak ) may be have the shaft in good condition  for more work

good luck

Hi Charlie, thanks for the reply.  I'm planning on getting a new big gear with the female threads and the new bolt to secure the new impeller.  I'll look around to see what I can find for a used pump body.

Re. the accidental hole, drill and tap for a loctited grubscrew? You may need to gloop up the seal where the edge of its hole is marked but that should be ok

I would source a replacement body.

As for the shaft, had the same issue but had all the parts in place to repair the pump. The shaft issue is a virtual certainty unless you know for certain it has been off before.

I wound up using the dura-fix (http://durafix.com/index.html) to fill the hole.  It seems to have worked, but the project is on hold for a while as I'm prepping for putting a new roof on the house.  I'll try to get some photos of the repair on here.  I'll report back after I get it back together and running.

Will be interesting to see the pictures.

If you end up needing a replacement body I can hook up and I'm just down in Washington.