May have been in the transmission section.
Boo will most likely chime in cause he is good at searching this stuff out, I am hopeless at it.
Now check this site out and double check this is what you have . It helps a lot if we are all talking about the same mower and if we all use the same diagram then we can refer to parts by numbers so we are all talking about the same part.
https://www.partstree.com/parts/mtd/mowers-lawn-garden-tractor/43995-136m660g062-mtd-lawn-tractor-1996-lowes/drive-frame-lower-pedal-assembly-wheels-rear/
It takes a few minutes to get your head around how the system works but once you do it all becomes blinding obvious.
SO pulley 74 puts a fixed amount of tension on belt 79 ( the top one ) by means of spring 66.
Pulley 82 has a fixed top & bottom, but the middle can slide up & down.
As this happens it changes where the belt runs, closer to middle or right out to the edge.
Because it is really two pulleys the net effect is the came as a drill or lathe with a pair of stepped pulleys.
When you put your foot on the go pedal you change the tension on the belt 25 and if the amount of tension you apply is greater than the spring ( 66 ) it forces the sliding plate up which makes the diameter of the drive pulley smaller so it turns faster and you go faster.
So first off you need to look at the spring and all of the pulleys.
The spring must be there and have not cut a hole through the mountings.
The pulleys must all spin freely & quietly .
next check the two belts fr wear and faults like cracks & burns.
The belts are the same width but the rear wears faster than the front cause it is shorter,
Unfortunately they must be replaced as a pair, which is a bit wasteful but that is how life goes.
Also important is the pivot point for the speed pedal 84, note it has 2 bushes, part 28 these wear preventing the pedal applying full tension so they should be replaced .
The same for bush 70 and check the hole in the tensioning arm it goes in. Same thing, floggs out oval over time. It is supposed to be lubricated monthly but no one ever does it.
If the belts need replacing they must be genuine or certified OEM replacements as they are not standard A or B section belts. I use Stens exact replacement belts down here because they are good quality and come for a better price.
Down here the pair of belts is over $ 200 ( Aus) .
That little lot shoud keep you out of the bar for a day or two, or drive you to drink.
The previous thread was a troybilt mower. not exactly the same as yours but the same system.