Most MTD mower used a copper clad aluminum 8 grade. One of the worst battery cables out there. Upping to a 6 gauge strand copper cable is fine as long you properly crimp on the cable lugs.
BTW you would to buy a complete wiring harness from MTD @ $350 if you want factory cables (MTD 629-0939). It is for the LT-1850 as the LT1650 (MTD 929-0943) is NLA.
I think you can figure which to go with that is the better deal.
Now some place will list the 929-0943 for sale but according to my distributor it is NLA.