Craftsman mower shifting problems

I have an older model 38" 5-speed Craftsman riding mower I bought used. Its been working well besides the transmission. At first, all gears worked great and shifting was no issue. Shifting began to become harder and it started skipping over some of the gears. Reverse then completely stopped working. When in Neutral, it was actually in 1st. In Reverse, it was actually in Neutral. I located the loose linkage bolt and tightened it just enough to fix the problem as well as lubricated the linkage and spring with a penetrating oil. This worked well. The problem is that the issue keeps repeating itself and I'm getting tired of having to readjust this.

1. I plan on using a threadlock on the bolt to keep it in place but I suspect there may be some damage inside the trans. Is there any way to check this with disassembling the trans? (Not a mechanic lol).

2. Could this simply be a result of a belt in need of replacement? I haven't replaced the belts since I've had it.

 

Water has run down the shifter rod and into the housing at the top of the case where the shifter enters (or that's what normally happens) and rust and dirt are making it hard to turn. That is why the shifter keeps working loose and possibly the rod is bending some getting it out of sync with the shift quadrant. Spray some chain lube around the area where the shifter enters the tranny and shift it back and forth, repeat, until it works freely. The chain lube doesn't evaporate as easily. Keep it out of the rain or it will keep happening.