There are two sizes spindle, from ducati 916-998 and ducati 999 The difference is the shoulder size.
I think you do have the 999 size, and can go with a 916 spindle, they are not expensive new, and might be the same as rusted used ones.
Ducati axles are 25,9 cm long
The older ones ( 916/998) have the 30mm piece 64mm long, the 749/999 have that 30mm piece 57mm long
1) Ducati 999 front spindle/axle: ducati part number 819.1.043.1A
2) Ducati 999 spindle nut: 748.1.014.1A
3) Ducati 999 washer: 8000.70119
I don't have 916 partnumbers, but you'll find out yourself.
999 picture:
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