You could try an old universal remote. That should do the very basic controls.
Be sure it has the code book ! A handy thing to have anyhows.
If the original hand unit is gone next step is a manual.
Since NEC was not a big seller & most were sold at big box & high end dealers
there were not many FSC's that would have had a manual subscription so
manuals would be rare. But with E-bay things do show up.
good luck
73 Zeno
LFOD !