I am not familar with Powermate circuit. Is this for field D.C. or is this the filter on the rectified three phase alternator output on an inverter-generator?
If so the alternator output voltage is a function of engine speed and load. My Yamaha EF3000i has an unloaded, at 3800 rpm engine speed, D.C. voltage into the inverter of 335 vdc. At full load it drops to just under 200 vdc. EF3000i is 120 vac sinewave inverter-generator.
I am guessing this cap in for the field coil supply. If it blew out because something else failed in the field regulator then just putting a higher voltage cap won' fix the problem. But it may at least let you further trouble shoot without blowing another cap.
You seem to have full drive on the field winding. Perhaps a shorted switching transistor that feeds the field winding.