have to agree with Kiwgf,
It is nice to have matching case halves, and you should check any machine for such, But not having both matching halves is not the end of the world.
Competent builders know how to align the critical surfaces. To be fair, Most of these early British bikes were built to very loose tolerances and quality control was not great.
Any casting has certain points that need to be in alignment but a great deal of latitude can exist in other areas.
I know of several people making replacement case halves and in some situations only 1/2 is beefed up or replaced. (IE: Steve Maney, Kenny Dreer). Dreer eventually did make both halves as well as heads, cyls and timing covers for his VR880 Nortons but I have 2 sets of cases of his early production and only the drive side was new.
It has been my experience with BSA, that they tend to get core shift and distortion over time.
On one 650 twin the Crank bearing bores were off side to side by 20 thou in one direction the cam bushes were off by 40 thou in another direction. When bolted up the cam and crank locked solid. Only after careful machine work was tolerances restored. I have pictures of famed tuner and tool & die maker "Sir Edward" Bilton-Smith doing the work.
I have also seen Goldstar cases shift (Matched sets from the factory) and well off the mark. The symptoms were abnormal wear in the cyl bores.
Dialed in on measurement it showed significant alignment issues. Ironically the owner of said machine in one case felt that the cost to correct it was excessive. He bolted it back up and Sir Eddy reported wistfully the fellow seized the motor on his next outing.
So, in my book, 50-60 year old alloy castings should be carefully blue printed whether they are matched case halves at the factory (With matching stampings) or mismatched halves pulled from parts bin shelves or swap meets/autojumbles.
The results of NOT doing so is predictable.
