key change between repetition bars?

[Marc Hohl]

I don't know whether this is kind of standard notation practice,
but I have to sty as close as possible to the original score, so
I want to achieve something similar to the attached picture.

I tried to work with

\version "2.13.13"

mel = \relative c {
 \time 3/4
 \repeat volta 2 {
    c4 c c | c2
  }
\cadenzaOn \key a \major s4 \cadenzaOff
\repeat volta 2 {
  c8 c | c4 c c c
 }
}

\new Voice { \mel }

but it seems that the cadenza part is way too big and doesn't dependent 
on the spacer.
Is there a way to include the key signature and tell lilypond to use 
exactly the space the signature needs?

Thanks in advance,

Marc