if you lost that much power check the condition of the cylinder wall, piston and piston rings. make sure to use the same blend of fuel and octane of fuel as before. if the engine sounds like it it revving high soon after you start off then your clutch is slipping and you will need new clutch pads, replacing the carburetor will not help although i recommend a cns carb and do lots of research how to tune it you will need to buy jets and a jetting hand drill to drill out the pilot jet if needed. you will see a big power incresase if your cylinder and piston/rings are in good condition. also be sure the carb is tightly fastend to the intake, is it possible you have a bad crank seal and sucking in air through crank seals.