One option is to look for some damascus barrels with similar pattern and good wall thickness as a donor and have it sleeved with them.

Another option is the half length subgauge inserts. They reduce the increased inertia (tubed guns feel way too heavy/slow swinging to me). A set of half length 20g tubes firing a 7/8 oz load at 1200 fps should give as much utility to that old gun as needed.