Welcome to OCC, you will be able to find a lot of useful information here and also a lot of helpful people.
I believe that you will find that the forward/reverse controls (the parts under the dash along with the handles) are very different between the 123 and 127. That said, it is my opinion that those controls on the 127 are far superior to the ones on the 123. I also believe there is a difference in the height of the dash towers between the two and I don't know that one will simply "bolt in" and replace the other. Anyone feel free to correct me if I'm wrong here.
Likewise with the steering column, although very much the same, I believe there is a difference in length of the center shaft and outer tube between the two machines. Once again correct me if I'm wrong.
The good news is, these parts are fairly easy to repair. Taking them out is something of a struggle, but once they are out, they can be taken apart and cleaned, worn parts can be replaced and you cam make them as good as they were when new.
The hydrostatic transmissions in the the two tractors are the same, but I'm guessing that making some repairs, replacing a few inexpensive parts and making a few adjustments will correct your "transmission problems".
The deck lift levers are also some different between these models, and I'm not sure if one can be substituted for the other or not.
For more information on differences between the two machines you can go here and look up parts and exploded diagrams for each one.
http://www.partstree.com/parts/cub-c...arden-tractor/
http://www.partstree.com/parts/cub-c...arden-tractor/
I also believe that once you whip your 127 into shape, you will be compelled to go through your 123 and make it a good running usable machine as well.
Good luck with your projects.