2007 Troy-Bilt Lawn Tractor Pony Transmission Issue

My troybilt pony runs, but refuses to move. This was my grandfathers lawnmower so I don't know it too well. I can't attach a video of what happens, but I did attach pics of the transmission and drive belts. Both belts seem to be on correctly, but could be loose. The drive belt looks good but the transmission belt is looking rough, I initially thought I was going to have to replace the the transmission belt, because it looked like part of it had been shredded. But when I broke it down, loosened transmission pulley, and cleaned all it out the transmission belt went back on and seemed like it was producing tension. But still, when I run the lawnmower, it refuses to move. Or will jolt forward a little. Sometimes the transmission belt stays on, sometimes it pops off. Is it just not producing enough tension after being shredded? I know from the repair videos (https://youtu.be/i5_ly-o3fOE) that I should not be able to get the transmission belt back on without loosening the transmission pulley. So maybe that's all it is and a new transmission belt would fix it? Or am I having real transmission/engine issues that I should take to a pro? And is there anything else I can try to do to indicate the problem?

Thanks for any help!

The top belt is not on the white V pulley.
It also looks like it is damaged.

There are special shape belts so you must fit original or OEM specification belts, standard mower belts will not run for long and may damage the pulleys.
It is also standard practice to replace them as a set.
If you just replace the top belt generally the tranny will not work properly through the entire speed range.