Perhaps, that is a drawing of aa earlier version. The whole project might have originally been a direct development of the MiG-21 analog, not only utilizing the the aerodynamic configuration, but the single engine as well . Maybe Later on it was decided to use two engines for reliability reasons.
In the new 'OKB Mikoyan' book, pp.247-8, the MiG-27II is described as the Mig-21K, and there's a photo of a _single_ engined model. According to this book, the Mikoyan OKB discontinued the project in favour of the swing-wing MiG-27, leading in turn to the development of the Su-25.
However in 'MiG: 50 years of Secret Aircraft Design', the Mig-21K is described as " an experimental version of the MiG-21bis designed to develop new on-board systems to be installed in cruise missiles..." ???