The width of the engine bosses on the 600/650/750 engines of that period were 2.5mm narrower than the later 900 cases.
Also, the swingarm pivot was a bushing of much larger diameter than the needle bearings of the later stuff. That works because all of the later engines have the rear cylinder reversed and will require a swingarm that accomodates the rear exhaust pipe passing through it. You will need to mount the later model swingarm.
I don 't know whether the Allazurra frame will accomodate a late model air-box either.