>But if a new 865 with the common mods will be closed a 790 with 904cc kit, I'll have a new bike with the same good performances of an old one modified.

I don't think it would.
The difference being cams & compression.
That said, your preferences indicate you might prefer the 865 "grunt" cams anyway.

Said Molly to James, that's a fine motorbike. Richard Thompson