As far as I can tell as I looked into it in a big way the fuel sensor option can be changed in developer mode of star, it's tucked away in there. I'm sure the two sensor installation occured around 2002/3 so it's my belief that some CLK's have one sensor and some have two!
Also the rear frame does need changed and I believe the cluster still doesn't sit 100% but haven't seen in the flesh to see it for myself.
The issue of mileage is also something that would need resolved, two options, buy a new cluster or buy a second hand one and have it reset to zero or there about to sync it to the ignition switch.