Scag three wheel rider drive problems

Scag three wheeler ride on. Not exactly sure of the module #. probably 20 years old. Changed the drive belt. Worked fine for 30 mins then lost power. Seems to still have proper tension. No leaks on hydraulics noted. Not sure if i am seeing things but the pulley on top of the drive system seems to shake when i run the engine. best i can tell linkage works. have it up on ramps now.