I've been busy and come up with a good way to install the second optoisolater needed for the CKP signal input on a Mk1, using the space that's free from not installing the VR circuit.
1) Install the second 4n25 in the lower six pins of U7.
2) Jumper U7 pin 4 to pin 5
3) Jumper U7 pin 1 to pin 6
4) Install a 0.01 capacitor between U7 pin 6 and pin 5 or pin 4
5) Install a 470ohm resistor between the top hole of R44 and U7 pin 8
6) Install a 4k7ohm resistor between the left hole of R50 and U7 pin 8
7) Jumper the top hole of R52 to SPR1
8) Jumper VRout to JS10
Please excuse the poor quality of the scan, but hopefully it's clear enough.
Nice and neat, and it saves you space in the proto area for other things.