My answer over here is probably better suited to your question than it was there. But that was about sistering, you're talking about scabbing (the other answers there may also be helpful).
What you are talking about doing is scabbing. Sistering is adding the same dimension board for the full length of the joist. Scabbing is acceptable for individual random joist repairs. It would not be acceptable if all 9 are consecutive joists. Scabbing should be done with the same dimension lumber extended 4' past the affected area. Then it should be BOLTED with 1/2 min. diameter lag bolts in a zig-zag pattern to eliminate shear cracks from forming. –oldhouseweb.com
There is no hard and fast rule on scabbing that I've found. The "acceptable" recommendation above is the opinion of that poster. Done correctly there should be no problem, IMO.
30+ yrs as a carpenter and don't know of any real rule of thumb. Call-out varies with severity of notching, joist size and span. I'd recommend qualified contractor or SE and let it go at that.
[scabbing] is installed which extends either 4' beyond the damaged section (or to a bearing point) or 1/3 the length of the framing member plus the length of the damaged section. –homeinspectionforum.net
That seems to be about it for recommendations on scabbing (they may be citing code, but I couldn't find it).
I might do one 2-by-X on each side before I had to match those four-by's. Either way, do a new load calculation and over estimate it: those old joists are now less than helpful. The lag screws I linked to in my other answer are nice (but you need a real impact driver to sink them) but if you go with 4-by's, you'll probably need bolts anyway. And since you're scabbing instead of sistering, I think bolts are in order, at least at the ends.
As for a fastening schedule, other than that sisters need two every 16" max, I'm at a loss. But here's a nice staggered pattern, with notes about distances from the edge, etc. (just ignore that it's a splice). Use four bolts like that at each end, and for simplicity, tack the rest of the old joist up to it with those screws. Again, be aware you'll need a serious impact driver for them or you'll be drilling holes just like for bolts anyway.
The only thing those old joists are going to be doing when you're done is hanging that ceiling.