One concern with welding is distortion.
Update: spoke to a very knowledgeable welder engine builder, he agreed that welding aluminum requires very high heat that might cause more trouble than its worth. He suggested to thoroughly clean the area reseal on the outside and leave the mating surfaces undisturbed. His thinking was if it wasn't leaking then the outside surface repair is probably going to be fine?
His point is well taken, and largely correct. When I had a badly-broken sandcast K0 case (note: the sandcast/K0 web is only about 60% as thick in this particular area: Honda made them much stronger here after approximately 3/1970 builds, and added a 3rd bolt, like yours has, after that) the biggest trouble we had with it was the heat applied in order to secure a good weld: this caused the area near the sprocket bearing to sag a little (about 0.010") so I had to have it progressively welded up, milled back, welded up and milled back 3x to make the seal good and the hole for the final-drive bearing round again (all that stuff took much setup time and machining).
Yours isn't anywhere nearly as bad as that one was, and your case is thicker and stronger there, too (all of them after 9/70 got thicker).
To be explicit, what would happen there might go like this:
1. The broken piece needed to be realigned by cleaning all the gunk and JB Weld (or whatever sealant was used) from the areas of the repaired cracks. This might (in 2 of mine, did) cause the broken 'square-ish' piece to come fully loose, which actually helped in one case.
2. The ragged edges (particularly at the sealing surface of the case) need to be cleaned away, usually done by hand with files and such, or more carefully with grinders of one sort or another. There will then be a small gap all around the 3 sides of the part that is broken out.
3. The part is replaced with the cases bolted together (using the crankshaft bolts at torque, and the smaller ones in the broken area, at 90-100 in-lbs) and tack-welded into place.
4. The cases are separated and the gaps welded in, and any clearancing needed between the welds and the transmission gears nearby needs to be done, also with small hand grinders.
5. Hondabond is applied to the resulting cases and they are fully bolted & torqued together (no shafts need be in place for this) and then the cases are separated to study how well the area is sealed. If the sealant at the damage site is not more than 0.5mm thick it will hold oil adequately: use Permatex #2 at that site if it is noticeably thicker there than elsewhere, and Hondabond for the rest of the case mating surfaces.
That's how I/we've repaired 3 of these, 2 being sandcasts and one being a late K1 (similar in web thickness to yours), and all worked out fine.
