You might try Northwest Short Line
Their online catalog, part 5 shows flywheels with a bore for the U-joint, just like yours. That's one approach. Lionel and MTH will also sell you a flywheel, or you could have one made at a local machine shop. The trick is having the right size bore to accommodate the U-joint. (Or you would have to change the U-joint on the gearbox end.)
If you're not absolutely wedded to DCS, an ERR Cruise Commander upgrade would allow you to run the loco under Legacy with excellent performance and 100 speed steps. No tach reader required!