HondaMan, I've heard a lot about your book. I'm on a tight budget though and have 3 manuals as of now. Gobs of special tools and some I just needed in general. Spent a few hundred and have bought a single part yet. Also i have to have the case inspected due to finding it was cracked and welded prior to me buying it. Bad welds too.
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YEOW! That is a poor job!
I suspect you may find it cheaper to get another set of cases than to have that one fixed up and re-ground flat enough to stop leaking again. Reason I say this is: while I have [once] seen a repair there where someone milled down that area where your plate is now welded in, so as to fit a rubber gasket over the area in the hope of sealing it (which did sort of work, to his credit), when the chain became a little bit loose (and he was running the normal 18T front sprocket, close quarters) it chewed up his nice little rubber gasket and re-started the leaking problem. He later swapped cases, but in the 1970s these were mighty hard to come by! Today they are quite available: you can often buy another engine for $50-$150 that doesn't run, and use the cases and all the other stuff as spare parts.
For a "perfect swap" scenario, find an engine with the first 2 serial numbers the same as yours (That is, after the "CB750E-" part of the number). For example, the "..23--" series was the first of the injection-molded cases, which became the "..24--" and "..25--" series cases, which were the most accurate ones made. If yours starts with "..23--" you can use either the "..24--" or "..25--" as direct replacements, as the ball-bearing grooves will all be in the exact same places. If you have a "..24--" or "..25--" case, you can still use all the way 'back' to the "..23--" case IF the case has the wide support area with 2 grooves for the output shaft bearing for the countershaft (nearest where yours is now broken). There are even a few of the last of the K2 cases "..22--" that have both grooves, too. These will give you the least grief in swapping them.
If your engine is earlier than the K3, or even a very early K3 (which was an "Old Factory" K2 leftover), the web where yours is broken will be thinner than in the later cases. These early cases have only one groove in the countershaft bearing area, and a narrower face to hold the bearing: these cases use a 1-row ball bearing and a different output shaft from the later engines. The later ones have a 2-row ball bearing at this site, hence the major difference. The last of the K2 and some of the early K3 engines had 2 groves at this site to fit EITHER the 2-row OR the 1-row ball bearing, with their different shafts.
All this said...you can also swap the output shaft for the "other one" if it came to that, and all the other parts will fit fine. I have some dual-row output shafts around here somewhere, from engines that otherwise died or were modified to use the 1-row bearing shaft, for racing purposes (it saves some extra HP at heavy throttle settings and high speeds).
If your engine is the 1975 "F0" or later, you will need cases as follows: for F0, you need F0 cases, sorry...for F1 you can use F1 or K7 cases with no troubles. For K7 or K8 you can use K7 cases: K8 cases will usually have fussier setup issues with the spacing on the mainshaft's bearing on the side opposite the clutch, so that can be tough for a newbie to have to live thru? For F2/3, use the black F2/3 cases, and those interchange fine. I have also successfully put F2 into K7 cases, with nothing more than some crankshaft bearing size changes.
Beware when buying cases: ask to see photos of the very same area where yours is crashed. There's lots of cracked and broken ones out there. Cracked can be fixed, broken is hard to do so - as you're learning!
