Jonem,
Firstly, we need to establish what bike you have (there wasn't a 1959 MkV). The most obvious options would be a 1949 Mk3 or a 1954 MkV - there are slight internal differences within the engine.
Going back to your problem - engine seemed stiff, then locked up - on old bikes with magneto ignition this can be down to the shellac melting in the magneto armature and then setting as it cools. On Mark series engines, this can be easily checked by removing the magneto and see if the engine then free's up. If it does, a mag rebuild will be required. Obviously, the ignition will need re-timing when the magneto is replaced.
If the mag is not the culprit, I'm afraid a total engine strip will probably be needed in order to check out the crankshaft/ main bearings. Worst scenario is that the crankshaft has twisted and will need re-aligning. The second possibility is that the front main bearing bush has turned and it's locating peg has distorted the main bearing flange to the point it has locked up the motor. This is the point where the 2 models differ - the Mk3 had a single bush inserted from inside the crankcase, whereas the MkV had a 2 piece bearing - the 2 halves being pressed in from either side. The later 2 piece design proved more troublesome than the earlier type (they have a habit of working loose in the cases) and a lot of owners have reverted to the earlier bearing. When replacing the main bearing bushes, don't make them too good a fit on the crankshaft - Mr Douglas recommended a running clearance of 3 thou - anything less is likely to result in another 'nip up' of the front main bearing (and another strip down) - when these bearing nip up's occur, there is no obvious sign of anything being wrong until the crank is removed from the motor.
Hope this is of some help,
Regards,
Eddie.
