Went back thru your post NL, found a side view of your machine when digging the drain line trough. When I looked along the bucket line appears there is a seam as if the bottom had been replaced prior. May be able to brace the side panels across the bucket and cut away that entire sheet weld in a replacement. May be the easiest on the wallet. Will need the teeth off yours, then order a cutting edge replacement with new teeth shank bolts and have the steel shop stamp out the bolt holes to match. From that point is just a lot of grind, 7018 and dress off time.
Looked to see configuration if same as Great Lakes has, look almost similar but not either.
Easy way to align the bottom sheet to the back wall is to set and weld some 'helper' small plates three or four top and same but staggered bottom then align the sheet between them until tacked into place, Wish I lived close enough to come help or make the offer to build for you.