For my M28 engine I recently bought a new carb with manifold because I want to change from double stage with adjustable jets to single stage.

I bought a Solex 34 PICS 5 carb but as I understand the gas cable system is different and I need to convert it?

Yes, from what little I know of these things the single choke carbs are 'pushed' open with a solid rod and the twin chokes are 'pulled' with a cable. It's not hard to convert it though, but you will need to use an 'organ pedal' throttle pedal which will mean making changes to the pedal assembly. Basically, you can make it work using all standard parts from a single choke 2cv, but it's a lot of swapping around parts, or you could probably come up with a home made system to operate the throttle with a cable, but you'll need to make bits yourself.

Well if you push on the topside you should be pulling on the other side, so I should be able to whiff something up.

I also bought it for the upcoming raid, but I'm doubting now... The current double choke is a bit more complicated but always runned perfectly, so it might be better to change after the raid.