What I would(*) do, if faced with such a situation: remove the exotic input connector; replace it with a metal plate with two holes for fixation and a central hole (plus rubber grommet) for the cable, and install a standard (Neutral+Live+Earth) cable, soldered inside.
Or, variant of above, pull out the prongs and drill a hole in the middle of the existing receptacle (again, grommet required).
Or, replace the odd receptacle with a standard IEC receptacle.
and get an IEC cable, commonly used for the mains connection of personal computers. An intermediate plate may be required to ensure there are no gaps in the enclosure (electrical safety)
(*) In fact, I am in just that situation with 60's vintage receiver-amplifier. Will wire a mains cord directly inside the apparatus.