Hi Stephen. Welcome to the last chance saloon.
Like all problems, the answer is easier with a little more history. Was it ever a runner, or someone's problem project? On the basis of your single post I can only assume this was a previously assembled and running engine. If it won't go back together now, it can only be a simple assembly problem. If parts have been replaced, that is the cause. If it's never run, then the basic assembly and set up is wrong.
With all original parts, does the camshaft turn, but the crank locks? Both locking up, it's crankcase mis - alignment. There should be a locating dowel where the crankcases join, above the camshaft. If the cam turns fine, the cases are aligned OK.
Usually some jointing compound is used between the cases. None, or less thickness than originally will make the cases closer together, and with a tight setting for the crank endfloat, this will trap the crank when the cases are tightened down.
Another cause is the drive side bearing, either replaced without setting the endfloat or the original drive side main bearing inner race moved away from the crank web, effectively increasing the overall length of the assembly. Closing the cases clamps the crank. If this seems the problem, assemble just the drive sleeve onto the crank, and tighten it down with the cush nut. This will move the bearing inner race back towards the crank and restore the original running end float. The crank should now be free to turn, if it was a previously running engine. Plenty on the forum about setting the endfloat the need to seriously tighten this nut on final assembly.
Have a look at the front of the cases, just below the dynamo cradle. Matched cases will have identical stamped numbers, side by side.
If the bike is a S/A variant, the three front primary chaincase screws can lock the crank if they are too long, the threaded holes enter the crankcase, rather than being blind holes. Earlier Plunger design does not have this feature.
Swarfy.
