How to recover/crack a lost ssh public key?

Question

SSH

There is a cunning Linux piece of iron router / skud created 4 years ago. Now flash memory is slowly starting to die, we decided to create the same new piece of hardware and transfer all the old data to it. But the public key and passphrase have been lost over the years.
Is there any way to recover/hack ssh?
Remembered that openssh 5.1 is used there.
Reading flash memory will not work.

Answer 1

The point of all this crap with the keys is to make it impossible to do in principle.
Don't piss against the wind.

Answer 2

muff.kiev.ua/content/ssh-vosstanovlenie-public-key...
And further on Google