New here... hi.
Ditto on there being no O2 sensors on the CB900F (even though DynoJet sell them with the PCIII!!!) I have 2 for free, except for postage, if anybody wants.
On the pair valves....
I took my pair valve out and hated the noise difference and hated the "soft feel". There is no difference in performance per se, but I do really, really like that popping from the Akras as I decelerate. It's a matter of choice, but you'll never have blue flame without the pair valves.
There is a set of covers that has the 2 indents for the valves - found on eBay (probably uk/eu sites). The solenoid plug can have a replacement female HM090 connector with 10k resistor across the terminals.