The K402 is larger ( in height/width) than a LaScala. Though I mean this tongue in cheek, it would squash a Cornwall.... but, probably sound good while doing so!
No...in my opinion, I'd probably investigate putting a smaller K510 inside the Cornwall and make it 2-way and yes, I think it would work nicely. If you could figure out how to stand the 402 atop a Cornwall, it would also sound good.
I'm not technical so don't "know" this would work....but think it would. The K402 is being crossed somewhere around 380Hz (when used on the LaScala) and the K510 is crossing at 420Hz on the LaScala.
Interestingly, the crossover points for the same horn on the Jubilee are 380Hz (K402) and 400Hz (K510). All of these references are using an active. I don't know if nor how it might change with a passive.
So, if the K510 will cross at 400Hz to a LaScala, I'd expect it to cross to a Cornwall (doesn't it cross at something like 500/600??)
As for what to replace otherwise, I have no clue. I'm a plug/play type. I go off of Roy's recipe's as they've done me fine so far.
Side note, I'm trying to nab a pair of empty Cornwall cabinets if I can. Awaiting a response email back from the seller. If I get them it's 510 all the way (though I suspect I'll sit a 402 atop them since I already have the 402's)