Does that tractor have a F N R shifter on the floor board in front of the seat and a 1 through 7 speed shifter on the right hand rear fender??? (YES or NO)
If you will post up couple pictures and the 7 speed shifter area for forward and reverse I probably can steer you to a owners manual that tell how to do adjustments. I have several manuals on file but need little bit of info to find you a adjustments manual.
Adjust the brake on that rig with a 1/2 inch wrench and just a 1/4 turn off will be the difference in Ok and too tight. Leave both hind wheel on the ground when adjusting the brake and when it's too tight the tractor will be hard to pus, then back of 1/2 turn and push test. The adjustment is critical for brake-no brake
Back the adjuster nut off 1 turn and see if it gets up to full speed.
It's quite common on that type that when the variiator belts get worn they won't act right.
Sometimes you can take your heel and pull back on the clutch brake lever and get it to go faster when the belts need replaced. They have to be replaced as a pair.
You should always kill the engine with the shifter in N and the clutch released so as the variator will get all speeds.
One guy posted the reset procedure, like start the engine place the shifter in Neutral and then slowly release the clutch.
Do not pay any attention if both 5,6, and 7 are about same spped of travcel.
Main thing is that 1-3 pull good with no belt slip up a slight grade.