Unless I have miss understood this there were two problems.
1...an incorrect 9 tooth starter was used for a while. The flywheel is an 8 tooth starter flywheel it doesn't like a 9 tooth starter. When this is done, this is not the first time somebody has used the wrong starter, it damages the gear teeth on the ring gear. The wrong starter gear will not match the gear spaces of the ring gear.
2...the starter relay froze on several times and the wrong starter ran on and on causing a terrible racket and more damage.
3...a new Nippon Denso starter has now been fitted. It has the correct 8 tooth gear but it sometimes doesn't engage all the way or fails to crank the high compression motor.
What Chas suspects is that the damage to the ring gear are preventing the starter gear from engaging.
I still think the flywheel is not damaged enough to cause his complaint. I think the ND starter needs brushes or cleaning. Maybe the battery is weak or the charging system is marginal. I could be wrong, that's just what I think at this point.
Those V marks in the gear teeth do not look like they are caused by any starter bendix. The damage to the gear teeth seems to be, to me, a little bit of banging on the top corner edge. This sort of damage is often caused by the correct starter.
Maybe somebody with a bunch more experience than I have will see more here.
I have a rather elaborate suggestion. Put a stock flywheel with good ring gear on the bike now. Reassemble and see if the problem is now solved. You'll have the use of a much slower bike for a while but then if the problem is solved by the stock flywheel you will know it is the problem