What Scottly said ^^^...
I have had similar situations with the CB750 cases nearby that area (chain crashed thru cases) and can only offer a little bit of advice that you should take to a KNOWLEDGEABLE and SKILLED welder with some form of wire-feed welder (not stick welding), as JB Weld can't take the torque a starter can dish out:
The case can get hot enough to warp and sag if the welding continues too long. This happens because welders like to heat up the whole area first so it is easier to lay new weld onto the site. Instead, it must be first filed (or ground) flat, then the bosses built up a little at a time with lots of cooling time in between weld sessions: lots of welders I know won't do it. If it is done by heating the whole case up until it is easiest to dab on new weld, the case can warp: then the case may have to be machined very precisely flat again so they will mate, or it will leak oil where they join - right next to the weld (they soften and lose some shape).
I might suggest this approach if you still want the electric starter: make up some small bosses (round or square) that will fit on those mount sites, then file those bosses to have a flat surface, then have the new parts welded onto those original bosses. Then hand-file them until they will mount the starter straight and in-line with the nose of the starter so it will fit perpendicularly to the face where the starter shaft inserts into the engine. Otherwise the starter shaft will sit at an angle and troubles will happen with the engagement inside the engine. After all this, mark and drill & tap the new bosses for the 6x1mm holes, drilling just into the old bosses for a couple of extra threads to help bind the starter, new bosses, and case together when the starter's mounting bolts are installed, This might entail using slightly longer (or custom-trimmed versions of even longer) bolts to mount the starter.
It is do-able, but painful. I usually only do this stuff on sandcast 750 engines, as they are valuable enough to support it. The last one I did (cases crashed with chain) cost more for the crashed-case welding-machining repairs than the whole rest of the engine's rebuild, and it started out as a mass of rust and dust from 40 years of sitting in a Texas shed, engine open, needing every "update" that Honda had for the sandcast engines, to boot.
