I'd be terrified to use a 4cyl flywheel on this motor. I don't really see the issue in using a gas 6cyl flywheel with a diesel motor though, yeah it might be lighter, but people have been switching to lightweight flywheels for years on all motors.Over here in NA, I believe you're going to be hard pressed to locate a manual flywheel for anything other than an M102, M103 or OM601. I don't know diddly about the 202 models, but I'll bet manuals are relatively rare in those over here, as well. Nearly all 124 w/60X manual swaps I'm aware of over here were accomplished using 201 four cylinder parts, including the flywheels.
On gassers, switching to a lighter flywheel reduces inertia, which makes the engine more responsive and more rev-happy.I don't really see the issue in using a gas 6cyl flywheel with a diesel motor though, yeah it might be lighter, but people have been switching to lightweight flywheels for years on all motors.
I guess I should've said it better: fitting a progressively lighter flywheel might eventually make the diesel engine not run (while it would make a gasser idle rough) - sorry, my mathematical mind tends to work in asymptotic analysis...I did have a heavily modified TDI back in the day and looked in to lightweight flywheels. While yeah, you make a point with lighter weight flywheels not so much making it more rev happy, they don't make the motor simply not run. They make the revs want to fall even faster compared to a gas motor because it is fighting compression. I don't see a stock gas flywheel being unbearable with drivability, i'm sure they're around 25lb flywheels and diesel ones are probably over 30lbs.
I'm in the same boat as you though, just my thoughts, i don't know. I'm hoping someone with experience can chime in.