Dave, I wouldn't cut a lower piece to replace. You can make a new one from 2" electrical RIGID, not thin wall) conduit, 2" aluminum, 2" stainless steel. DO NOT forget to cut a top slot to align the piece(assuming the pin is still there, and the 5 to 7 1/4 holes to the sides of the top center for gas to escape.
Having said that, if your pipe is able to be removed (like those set screws are going to not break off...or the rust between the pot and the pipe can be freed up, you will be lucky) IF that can happen, soak the hell out of the screws with PB blaster OR heat them to remove them. A "Fire Wrench" may be necessary to have on hand, too. If you don't have any anti-seize, get some for the replacement journey. You may be able to remove the 'pot set screws' on the motor side and the fines lever, then take out the pot assembly if it proves to be impossible to get enough work room. If you have an older style base, the chances are 'slim to none' about unbolting the side panel.....but heating the nuts would sure help.
I'm guessing that the pipe rotten out, then was further destroyed by abrasion and acidic effects of some wet coal...WAG....or something is or was misaligned.....
It's not a great time to have to do this, and today will be a "Give a Gift of Skin" moment if yours is typical.
I may require an adult beverage or two to get the heart rate down after the stitches and transfusions.....
