if you don't have a manual for that machine, look in the cavity under the tilting seat - get full model number and serial number.
go to PDF Manual Web Archive - enter data, download manual(s).
If tranny is servicable, details s/b in manual.
OTW, ck belt, tranny input pulley, control linkage.
Last resort may be removing tranny, turning upside down to drain old fluid and re-filling. Tranny oil itself can break down with time and excess heat...
might also try the forum's search function to ck for similar threads....