I'm about to get a Digitech Valve FX, but it doesn't have a fx loop, only left and right outputs.

At the moment im in a loss on how to connect it to my ENGL Blackmore.

Any help is welcome, thanks guys.
The effects unit doesn't need a loop, you run it in the loop of the amp.
Connect the amps FX send to the effects units input, and the output to the amps FX return.
Check the manual of the effects unit to find which output (left/right) is the mono output if it has one. Thats the one you should use. If it doesn't list an output as the mono output, just use the left output.