HI, hd_lonewolf
I am assuming you are referring to the later separate A or B series boxes????
Most were fitted with BSA's own 6 spring clutch
Convertion centres are readily available to fit 4 spring Triumph type clutches, these were fitted as standard to the last of the A and B bikes
If a splined mainshaft from a plunger framed semiunit box is fitted I believe a Norton Clutch will fit
There are many belt drive clutch conversions around, search the net!
Also Phil pearson sells a chain drive Suzuki based clutch
It all depends what you are up to with the BSA box and how much money you have to spend ???? ???? ????
Triumph clutch conversions go from about £250-300, Pearson £425 upwards, belt drives ?? as much as you like
HTH
John O R