Oh, God, he's still not listening.....it's not starter motor, it's not sprag clutch........so let's see, what's left?
There are two mechanisms you need to consider:
1. The sprag clutch
2. The transfer gears.
Dealing with the sprag clutch first, it's a simple enough design- a circular array of rollers allow "free-wheeling" in one direction but locks in the other. It allows the starter to push the engine round ( to get it started) but the engine can't push the starter motor round. Same game as the free wheel in a pushbike rear hub.
You also get a complicated version in some designs of slipper clutch, and a heavy-duty but basic version in a lock-up clutch for drag racing.
The transfer gears consist of two gears locked together- one is fixed (welded, usually) to the shaft going through them both, but the other is just pressed on.
The second, "pressed-on" gear, has merely an interference fit onto the shaft going through them both.
This means that, should the crankshaft be held in place due to a mechanical problem, the gears will rotate, one against the other, and prevent the starter motor stripping the teeth from the transfer gears, the starter motor pinion or sprag clutch outer gears.
The starter motor generates lots of torque, and so it is necessary to have a way for excess torque to be released before damage occurs.
But whenever this takes place, the second gear wears the interior of its mounting hole, and/or the shaft it mounts on wears a little, and so the torque required to make it slip decreases with wear.
This results in the gear slipping under normal usage conditions.
To check for wear:
Firstly, strip out the sprag clutch.
Occasionally, you might see the the rollers, instead of being round, have flats worn in the outer circumference of each roller.
If you can't see any out of round, replace the transfer gears with known good ones.
So, as per my diagnosis, the sprag clutch is still functional, but the wear on the transfer gears means that they cannot now deal with enough torque to turn the engine.
This is my final word on dealing with your issue, without wishing to cause offence you seem to be a bit hard of understanding.
FFS, just replace the bloody gears and let us move on.
Being in denial doesn't get your bike fixed.
If your "mechanic" can't fix it, give the bike to someone who can.
He might suggest that you have the two gears welded onto the shaft- ask why, if that's a good idea, Honda didn't do that in the first place?
Or, I'll give you £300 for your bike as is, fit new transfer gears and sell it on for a profit; you can buy something else.
I did exactly that with someone else whose "mechanic" replaced the sprag clutch with a new (expensive) Honda one, then couldn't understand why it still didn't work. I had it fixed and running in an hour......and sold the new sprag clutch into the bargain.
Oh, and I have a 2nd hand set of good transfer gears for £40 inc postage to the UK. If you're outside the UK, suggest that you buy the Honda ones.
Lastly, please don't stick your old ones on Ebay.
It's not falling off, it's an upgrade opportunity.