Aye Ian, pleased to read that the diagnostics are not failing you.
As for the "Dutch company....."? That wouldn't happen to be HPI from Belgium?
I hope that you can get the carb sorted, sounds as though coming off the pilot jet and onto the slide is the problem with the bottom end stumble, whilst the top end could be that Gold Star "silencer" having too small a diameter hole in the internal core. An Armours one, of similar design, throttled the mid to top end of the Ducati for that very reason.
Treat it like the Norton ES2 and let it breath!

Good health, Bill