You will not necessarily change the ring and pinion out unless you buy an aftermarket set that is sold by either Vogel manufacturing or Midwest supercub. As far parts used from Mopar you can use the 8 ¼†carrier, you can shorten a set of axles for this if you want but you will end up using a smaller bearing size with the Mopar axles or you could buy a set from Midwest Supercub that will utilize a larger bearing. Less friction-stronger axle. You will have to bore out the axle housings to accommodate the different bearing for the axles. You can buy a set of bearing cups from Vogel or Midwest. You will not use a Mopar ring and pinion. The tranny gears you can buy from a half dozen different places. A hardened top shaft is also available from Vogel or Midwest SC. Also changing out the front bearing holder for the pinion shaft would be a wise idea. Are you wanting to use 2, 3 or 4 pulling gears? You will have to decide this before you buy the appropriate package when you order your gears. If going with a 4 pulling gear set up, you will also have to come up with a different set of shifting forks. Again, Vogel or Midwest SC sell them. I made my own but most people don’t want to mess with that. What motor are you putting in this tractor? Which class? If you are going to pull in a class that requires a full on pulling transaxle then you will also have to make clutch changes. Depending on which class you are pulling will determine which clutch to use. If you are using a stock stroke motor then you can use either Vogel or Midwest SC. If you are pulling a well built 50ci motor then I would recommend using the Vogel clutch only, I have smoked the 3 and 4 puck MSC clutch. I may give the 5 puck a try if I don’t have the Money for the Vogel clutch. As far as using the old Cub Cadet 70 there is absolutely nothing wrong with using that tractor. My first tractor was a Cub Cadet 100; the only difference is the motor between the 70 and 100.
If you want to put the 12hp. motor in that frame the only thing you will have to come up with is the correct oil pan for the 3.25 stroke Kohler motor that fits the Cub Cadet Frame. Vogel and Midwest SC sell them. After that the motor will drop right in the frame and bolt up.