I believe your tractor is equipped with an 18hp Briggs and Stratton engine. You should be able to have a look on their website, and find an engine manual for it, which will include this information.
I'm having little luck determining the engine model with the resources at my fingertips, but it should be marked. The Briggs website will help you locate the numbers required.
The diagrams I found implied that the engine is an L-head. These engines generally don't have an easy way to adjust valve clearances, beyond grinding the stems. In my limited experience, they don't need adjusting during the life of the engine. The exhaust clearance will tighten up if the seat begins to erode, though, and at that point, the engine is generally nearing end-of-life.
I assume you've tried the carb adjustments listed in your tractor's manual? It's available online if you don't have a hard copy.