The radio will be so old it will not have the capacity for a MMSI, so you don't need that. It will not have a federal call sign, like WDH 2400 or so. So like 99.9% of the operators out there, just use your boat's name as the call sign.
IF you buy a new radio capable of DSC
which also has the digital emergency
broadcast, you will need a MMSI. Boat
US will give you one for free, but it cannot be used outside of the continental US. If you plan to go international, you will need a MMSI issued by the FCC and a ship radio license--all about $160 for ten years. Otherwise, the free Boat US MMSI will be fine and you can easily input it to your radio. Higher end radios will also have their own built-in GPS
which will add lat and long to your emergency
broadcast. A good feature.