If a stepfather adopts his step children they are issued with an adoption birth certificate with their new surname name on can they now use that one instead of their original birth certificate ?

Yes.

Yes, the new certificte will show the adopted father as the father. This certificate will now become their new official birth certificate

** poud father of a wonder 30 year old boucing adopted son Charlie

If the children are legally adopted. Their altered birth certificate will entail their legal name.