GT 3000 is not the numbers we need to check the muffler as they were available for a long time .
To get the parts breakdown we need the model numbers from the tag under the seat.
Mufflers are usually part of the mower , not the engine as they have to fit within the body.
So you should be chasing Sears for a muffler for you mower.
Having said that any Kohler single cylinder muffler will fit, but of course you will need two of them.
How much of the old system do you have ?