You are meant to fit a 38mm PHM carburettor. You can see the exact carb in my Fcaebook/Taffmeisters/videos of "timing an FE600e (1998 )".
If tuning I would have gone for the 40mm though. 40mm was fitted to all Husabergs 2001-2003 except the 400 and the jetting will have to be altered to suit. Suggest you keep to Husaberg as they were the only ones that constructed the internals the way they did.
regards
Taffy