Pretty sure your tank is the same as 600, 750 and 900 SS models from 1993 - 1998. That should broaden your search, as the 400s were pretty rare.
To be honest - what you have is a 750 with a 400 motor in it. That is - looks nice, but won't go.
Consider investing in a 750 before you invest too much into your 400. The 400s were really only built for the Japanese market (most here were 'grey imports' from Japan) as they were a big market for Ducati at that time.
The 400 used a combination of '90s parts and older Pantah engine parts. It was all about getting under the Japanese under-400cc registration cut-off.
The earlier 400s used small carburettors similar to Kawasaki four cylinders of the time. The later ones used the (too-large) Mikunis from the larger models, and were virtually impossible to get to run properly.
Not trying to be negative as much as forewarning you..
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