Without more information this is slightly tricky.
Assuming this is a shunt dynamo, it will motor in the direction that it charges. Reversing the supply polarity will not affect the direction of rotation, it will only affect the polarity when it builds up as a generator.
If you dismantled it then it seems almost certain you crossed the field leads. Now you have put that right you seem to be part way back to normal. The most likely reason for not building up after you flashed it and got it running is that you have disturbed the brush bedding in the dismantling process.
Not knowing the state of the commutator it is difficult to comment, it is old and the original brushes could be hard carbon, which is a bit mixed in its properties.
Check that the commutator has not worn down to the point where the mica is level with the copper bars. If the mica is still below the surface, the comm is smooth and without flats on some bars you may be able to just re-bed the brushes with glass paper wrapped round the comm.
At that age you may have to resort to a skim of the commutator and undercutting the micas, quite a skilled job, if you do it, make sure the commutator is skimmed dead true with the shaft, under cut it carefully, removing all the mica at the sides of the cut and remove any flash from the edges of the slots as this will "flip" the brushes just as badly as any projecting mica.
If it has the original hard carbon brushes and they are well worn you may be better off changing them for modern electro graphite ( these look like black carbon, not shiny graphite).
Now you have got the field connected correctly and made it build up by flashing it will have residual as long as you haven't dismantled it. Try running it up to speed and press on the brushes with bits of wood, quite likely it will build up unless the brush bedding is really bad.
Not seen this post until Allan replied. If you are in trouble after trying this then take up his offer it may need a bit of backwards and forwards correspondence especially if it does not turn out to be a plain shunt dynamo.
Flux