Welcome Lars,
I've done a similar 3750 ship restoration, I think everything except the cannon is available as a DS order direct from Playmobil, so once you have it in your hands work out what you need to replace, make a list with descriptions and the part numbers, and just order the parts direct from Playmobil.
You can check Playmodb for the parts and their numbers, and don't forget to check the 3053 set as well, as it's nearly identical.
I hope you and your son have many happy hours playing with your re-discovered Playmobil!
cheers, Martin