Older briggs and stratton MTD problem shifting between drive and rever

Hi, i just bought a riding mower i think it was made somewhere around 1990. i have completly clean the carburetor and bought a new battery the engine works good now but i have a problem shifting gear in the middle only got to go in reverse once and in drive and now its stuck there. Here's the model #142-849H515, i removed the cover in the middle and the lever to the transmission/transaxle is connected.

Engine ON or OFF its doesn't want to change anymore. Before i got it started i could shift between F/N/R no problem and the clutch was really hard to press down but when i got it started after cleaning the the first time when i release the clutch it wouln't go foward or backward in drive or reverse and then out of nowhere the pulley above the transmission started to spin (i think it was seized) the clutch was back to normal but now its really hard to change from F/N/R and its currently stuck in drive. Also the shifter for the speed 1-6 is not moving anything when i look in middle. Any help would be greatly appreciated.

thanks