Wow, looks nice
and ... well ... pretty used too.
There are different rims mounted? Have you made a decision here, I personal would take low bead rims out of aluminium from Morad (former Acront), these are lighter.
A close pic of the drumbreak surface of the hubs would be nice too.
To the deassembling of the engine, to fill the piston / cylinder combo with WD40 or something better for a week or so is a good idea, I would then first try with heat, heating up the cylinder, then I would only try pushing not using any punch force. If this does not work, rebuild a plug, take out the ceramic stuff, weld a grease nipple to the plug and then use a grease gun, works very nice and you will have the piston out without damages.
Maybe the piston is different to any standard Bultacos you knows ... ?