G'day TC and Bon_Scott
That image of the reel sprocket is a great example of poor
chain adjustment in the mower's history.

The sprocket is past serviceable life (as you say), and the primary chain
must have been un-lubricated and 'flappin' in the breeze' to cause this.

Note the difference in wear between the driven sprocket and the
smaller driving one (to the landroll clutch).

Quote
The chain linking the cutter to the engine is a solid piece - no split link. Could this be an issue and am I right in thinking it may be non-genuine?
Yes TC, not genuine, and not recommended.

Reel bearings should be replaced with new sealed bearings
as a matter of course.

Please keep us informed.

Many thanks
-----------------------------
Jack