Just to round up my myriad comments into its own answer.
To me, it looks as though all the previous 'remodellers' did was to remove the decorative end-cap mouldings. They didn't damage the actual treads, though one of the lower ones has damage to the nose, likely unrelated. Fortunately, the mouldings are not structural in any real way.
This leaves the balusters to flap in the breeze a bit. Structurally, they just need tacking back into place with a couple of judiciously-placed nails [into the treads], then they're trapped in place by the new mouldings.
The original end mouldings will likely have been factory made, each as a single piece, for rapid fitting on site. An L-shaped piece, router-cut on three sides. Push into place & nail on. Note there are no visible nail holes in the baluster or stringer fronts, which adds weight to this theory. Also note that the moulding fixings are into the tread ends, not to the stringer or balusters, so they float correctly with the treads under load.
I don't know of anyone that makes that L-shape any more - though you never know. [Though I have always lived in Victorian houses, they've all just coincidentally had closed stringers.]
The modern equivalent would be to get some bullnose-type moulding & mitre cut three pieces to make up the L-shape.
You could also perhaps get away with this used to fix the damaged tread front. Screwed & glued properly it should stand up to the traffic.
For somewhere to start on finding the right moulding, try https://richardburbidge.com/shop/mouldings [rather than B&Q's limited selection;)