Voltage regulator

Newbie to this forum. I have a 1980 XS400 SE, that doesn't seem to be producing a charge from the stator coils. Everything checks out fine, no breaks or shorts, and I suspect the solid-state separate voltage regulator is not allowing any feed from the field coil (it appears to be on the output side of the field coil, the input is fed from the battery via the ignition switch circuit). Can it be tested at all? They are like rocking horse sh*t to find, and most of them seem to come from Germany or USA at ridiculous postage cost. Can I replace it and the rectifier with a more modern single-piece rectifier/regulator, and if so, which one? And are they relatively straight-forward to wire in? Any help would be gratefully received.