Here's the rub: a proper midpipe with resonator is nowhere to be found at any of the quality restoration parts suppliers. What should I do?
First be happy they have not required it in the past
Going forward I would suggest you make the best effort you can, possibly improving, if possible other details and live with the deductions knowing you did the best you could and hope that your choices will increase. Can the hangers or hardware be better? Can cleanliness be improved? Things like that
Of course if you did have a perfect or correct system you would want (and judges should want you to be) to be rewarded for that detail as much as anyone else would.
Without a need, apparently no one has choose to make an acceptable reproduction and often the need/demand and the supply/availability are interconnected.
Look to other areas that you may have been deducted for in the past and maybe improve those areas to offset some of the points.