You could try a new needle and seat.
The carb might be cracked internally from frozen water in gas. Very rare but sometimes there is nothing I can do to repair a carb. Used parts place or Ebay for a replacemnt.
If the needle has a rubber tip, then the seat is solid metal. You should be able to buy a replacement seat for it. It is tricky removing it and pressing new one in.
If the needle is solid metal, then the seat should have a rubber seat inside the brass bushing. The needle and rubber seat should be replaceable.
This is how I check the needle and seat. I am not sure what the carb looks like on that 21hp but this is how I check on smaller engines. With carb off, inline fuel shut-off to closed. Remove carb from block. Remove bowl. Blow the needle and seat dry with air. With carb upside down(bowl off), turn fuel on. Watch for where wetness is coming from.