These are the most annoying faults, turning a simple modification into a hammer thro' the tank moment. So start at first principles.
Dynamo was working fine, so a headlamp bulb or meter across a D-F link to earth establishes whether the dynamo is still giving any output. As TT notes, field coil connections/integrity are number one suspects, also dynamo body earth connection also something to consider. Service Sheet 809 gives basic test procedure.
Big spark from battery pos to earth shows for now the battery is good and battery earth connection is fine. A test meter from your pos jumper to F should indicate continuity if all is in order. Maybe you are expecting a BIG Spark.....as the current is fed through a coil with resistance, only a smaller one is to be expected. Could be all is as it should be, plenty on the Forum about waking and repolarising reluctant dynamos.
Swarfy.