Make a new cab, same width, depth and "wooden rails to hold chassis" as current head, reserve same internal height for amp, including iron, tubes, etc, and add enough for a front speaker panel, same width plus 12.5" for speaker frame plus 2" height so you can vertically center speaker and have *some* wood joining left and right halves (most of the center will be the 12" speaker hole).
It will be a very compact and luggable combo.
Optionally you might add extra 1" or 2" height, your choice, combo will still be compact; and/or up to 5" extra depth if yopu want sound slightly fatter but combo still fitting in a standard car trunk with room to spare.
If in doubt make it 1/2" too large than 1/2" too small.
EDIT: I would turn both channels (which as-is have no sparkle at all) into 2 standard Blackface "Normal/Guitar" preamps.