When I removed the crankshaft assembly from my '74 850 the inner races of the main bearings stayed with the crank. Normally I'd say they were broken.. but according to the FAG data sheet the bearing is listed as "separable". Is this normal, and can I reassemble the engine with the old bearings, or do they need to be replaced?
Yes, the main bearings would normally be "seperable".
However not in the way you describe, as the correct 850 FAG NJ306E cage and rollers should be captive to the outer bearing race (NJ type bearing) with the inner race as the seperable part.
The Norton factory used both NF and NJ bearing types before they introduced the heavy duty "Superblend" RHP 6 MRJA30 main bearings on the 750s, and the FAG NJ306E on all 850 models.
The factory issued a service bulletin recommending that only the NJ(306) double lipped outer and single lipped inner type bearings should continue to be supplied and fitted
All 850 models should have been supplied with the FAG NJ306E bearings, and that bearing is still the best one to use for all 850s and earlier 750s far as I know.