Did my '67 Bultaco 250 Pursang crank several years ago. Things I remember: The pin to crank weight press fit is extremely agressive. My friend's 10 ton hydraulic press barely got the job done. I pressed the weights partially on the pin, roughly adjusted the phase by beating the weights with a lead hammer and checking with a straight edge, then pressed the weights on the rest of the way. I mounted the assembly in a trueing stand and checked radial and axial runout. It took me a couple of hours of beating the crank with a lead hammer, checking, beating, checking. It's amazing how hard you have to wallop the thing to get any movement at all. I finally got it running true within .001 in. I also replaced the crank bearings and seals. It ran perfectly smooth when done. I would not use a brass hammer. It will ding the crank. Use lead (best) or a rawhide deadblow. Wear gloves.
Time goes by like pouring rain, and much faster things...