Hi Tim.
To answer this I would need some more information. First off you can't buy a new Saginaw box, they've been out of production for 20+ years. There are some "new" boxes on the market but the ones I've seen I wouldn't use on a lawn tractor.
I can bring any original box back to life and better then anyone's on the planet. The biggest concern are the gears. As mentioned if they were incorrectly adjusted they will certainly tighten up the lash but too much will wear them out. It's not as much mileage as it is condition and setting.
I've had boxes with under 40k miles that were adjusted and needed new gears and I've had boxes with over 100k miles and 40 years that had good gears. Once the gear condition is known then the rest of the internals are changed and the box is setup to blueprinted spec's. I use my own spec's & machine fit the parts.
A quick way to tell is to look at the lash screw in the cover. If there are 2-4 threads showing that is a good sign. If there are none then the gears are most likely done.
Did you get your old box back? I would ask to see who rebuilt it as well. I have repaired MANY boxes that were rebuilt before by both owners and vendors. I even kept some of the more unsual parts I found in them for paperweights.
Examples of what I found opening up rebuilt boxes:
1- no grease
2- upside down rack on the ball screw
3- Welded teeth
4- Welded rag to input
5- Too much grease
6 - Painted only, no internal parts replaced( very common)
7- preload and lash not set correctly,many times worse then a 100k mile orginal box.
A std rebuild box without new gears should be in the $200 range. With new Gears add $250. A custom box figure about $100 more. Many times people get the box replaced but have no idea what it is or what was done. This has lead to the thought these boxes are no good and never were. That the aftermarket boxes and racks are better and that simply is not true. I'll take a blueprinted box with good rag & suspension parts over any aftermarket setup. They fit, look better,hold the value of the car and cost 1/2 the price.
Good luck.