I've lost the registration # of my trusty SF6 and need to buy the programme again, yes, I have later versions, but 6 has things that the newer ones don't have.
You should be able to find the serial number of SFP6 by going to My Products on your MAGIX My Account. Even though 6 was released by Sony, customer data was transferred over to MAGIX at the time of the change of ownership.
It is highly unlikely that you would be able to purchase SFP6 through MAGIX - only the latest version (15) is available, but you might find one for sale on EBay or similar - but do be alert for pirated or scam versions.