Not familiar with this tractor and not knowing what engine is on unit, i can only suggest a complete carb rebuild or replacement. The fact that the engine hunts and surges is usually a carburetor fuel mixture problem. By the sounds of it, probably plugged in the idle circuit do to the fact that the surging is at idle and that the idle mixture effects high speed performance and that the starting enrichment from the carb usually comes from the low speed circuit and the transition circuit.
If it is too lean a condition partial choking of the carb may make it run smoother. If it does then thats it.
There are many factors that can cause poor performance and if you go thru and check the health of your compression and ignition components it wouldnt be time wasted.
Good Luck Jim

Rebuild kit is $70 and a new carb is $130 when I priced them a couple years ago.

Just grasping at straws here, but-
Maybe a valve adjustment if this has mechanical lifters. IF the lash has gotten too excessive, the engine wouldn't "breathe" properly.
They would have to be at the point of being excessively noisy though-

Check intake system for air leaks, including the carb throttle shaft. Air leaks lean out the mixture which can cause surging/loss of power.

Check compression.

Tractor will start and stay idle but when I engage the blade it runs for a minute then shuts off. Any suggestions on what the problem is?