Congratulations...the 123 is the first of the hydrostatic transmissions and a very solid tractor! It would be any one of these three, and if it was mine, I would do all three of them:
1st: Change the hydrostatic filter on the underside of the tractor toward the rearend.
2nd: Remove the rearend cover and drain/clean out the old gunk in the transmission.
If the unscrews (you may need a filter wrench to assist getting it off). When you replace it, clean the space around the fitting to insure a snug fit. Fill the new filter about 1/4 of the way full with HyTran hydraulic fluid. The Hytran has great water absorption properties that a lesser brand does not have.
The back transmission cover comes off. Be sure to have a large container/reservoir to catch all of the fluid/gunk that comes out (about 8 -10 quarts....sorry, I forget this one). Clean out with cleaner. I try to not use paper towels to prevent pieces from breaking off and causing a future clog. Replace the gasket (if a cub dealer is not close, you can purchase gasket material and make your own. Use gasket sealer on both sides, replace cover, torque bolts tight (Tech Section contains manuals for your cub cadet 123). Refill your transmission with Hytran until it overflows out the rear plug (again, an overflow drain pan would be needed).
Sorry, this is so long....Others will chime in with far more knowledge about this matter than I have. Great to have you as a member....especially with one of the finest cubs, the 123!
Once you get comfortable on the forum, please post some pics of your tractor and equipment!
Cub Cadet 123