You're making your life more difficult than it has to be. There's a single step which, once done, will make the rest of the engine work a breeze...splitting the cases. X2 on what Adam posted. WTF is a ziptie doing inside the crankcase and what other surprises might be lurking?
Going through the oil pump, i.e. checking for binding, then removing the cover to inspect the gerotor assembly is a 5-minute job, if you work slowly. The timing chain assembly (chain, tensioner piston cap, idler, roller) should be replaced at this point, anyway.
Once the cases have been split, you'll then be able to inspect the internals, clean the case halves and have that missing aluminum replaced, the hole redrilled & tapped.
I'd go in expecting to find worn shift forks; those are easily replaced. I'd also want to inspect the crankshaft assembly, to be sure that the rod is not worn beyond service limits, at either end. Wet blasting, a.k.a. "vapor honing", can only be done on bare case halves. In some instances, it is the best option but those are a small percentage...imho. I'm not a fan of that process for CT70 engine cases as it changes the as-cast surface, permanently. Dry blasting, using walnut shells, is the preferred cleaning method; they're softer than metal, will take the paint off and not affect the surface. Any staining left behind can then be cleaned with aluminum prep, or sodium hydroxide (found in most oven cleaning products) and a nylon bristle brush...then rinsed with water. If there are stubborn stains, from deep pitting, that refuse to cooperate, then wet blasting would be the best option...but...only if the discoloration is in visible areas with the engine installed.
As for the case repair, once the casting has been cleaned, it's fairly straightforward. I'd prepare that spot by carefully glass bead blasting, keeping that tightly localized to just where the weld bead will go. Follow that with solvent cleaning (brake parts cleaner) and the welder will have a good surface that won't contaminate the weld. Contrary to popular assumption, heliarc welding heat, of aluminum, will be localized. A pro (and this is strictly a professional repair) will have no trouble laying down a nice bead of aluminum, leaving just enough of the intact side of the threaded hole to allow you to reestablish it. After the welding, the surface gets milled flat (ideally) or, if you're skilled enough, filed flat. The hole then gets redrilled on a press. A machine shop can do this, using a Bridgeport. Figure $35-45 for each repair (the welding and machining). The repaired case half can then be metalfinshed, if you wish; it's not a visible location, once the chainguard is in place. That's it...properly repaired and still the original case half with its SN intact.
What you might not realize, yet, is just how little time all of this really requires. An engine rebuild is roughly a daylong (~8 man hours) process, most of which is spent cleaning. The teardown & reassembly, together, comprise less than half of the total work time. Might need a week, or two, to source all of the parts and get the farmed-out work done, then it's down to a full day's work...to get the thing done right, back to mechanically-new, ready for the next 4 decades...and you won't have to go back into the motor, after hoping it will be okay for this season.