For the license, you will have to take the complete exam for an original 2nd Mate. This is the current commbined exam for 3rd Mate and 2nd Mate, it is almost identical to the exam you originally took for 2nd Mate.
You can revalidate the license without STCW. For STCW, you will need to complete STCW basic safety training, advanced fire fighting (unless your first license for 500 tons or more was issued before 1987), bridge resource management, and an “STCW Refresher” course. You can take a one or three day radar observer refresher or recertification course, you don’t need to re-take the original course. If you do not take ARPA and GMDSS courses, you will be restricted to vessels without this equipment.