Why would you need one hundred percent infill? If the top isn't enclosed, it's because your top layer settings are incorrect, not that the infill settings are incorrect.

It would probably be difficult (but not impossible) to complete the top and it may be possible to determine the failure point from the g-code if you know the exact layer height at which it ceased to print. A complication would be if only a portion of the last layer completed as there would be either a single layer gap in part of visible section or the printer would attempt to print over existing material.

If you've turned off the printer, there's a complication of determining bed zero and homing and I suspect that it's going to be very risky to attempt a continuation.

It would be easier to determine where to start the print by measuring it, then cutting the original STL at that point (Meshmixer, Edit, Plane Cut) and print the remaining portion, with the correct layer settings. Sand off the irregular, partially printed layer of the big nut and glue the two together. You'd have to sand the glue joint to make casting practical, of course.