Trying to chase up a 12 rivet output shaft for my 1985 RS
to replace original, cheers

If you're struggling to find one, don't discount the six rivet ones. Maybe it's a quality control or rider style thing, but they don't all fail.
And, whilst you're in there, consider swapping the crank for a later one - it might be available along with the output shaft. The later cranks are much lighter and make a much better engine imho.

Yep, replacing the worn parts might last longer than a used twelve rivet unit. If it's noisy and the cause is known, that might help with a decision.