if he is talking about the pto "clutch"-
and assuming replacement with a new one, it is a simple task if it takes more than 30 mins something is wrong
I just did the STX38 recently.
Remove the mower deck
Remove the gas from the tank
Push the mower over on its side
Remove the spark plug
put some nylon rope into the spark plug hole (use a LONG piece)
rotate the shaft until the rope is compresssed
now turn the crankshaft bolt and it will come loose
disconnect wire from above deck
clutch will come right off
push wire through deck
install new, tighten bolt!
REMOVE NYLON ROPE BEFORE TRYING TO START
(after turning mower upright, let it sit for 10 mins or so to be sure all oil returns to the bottom of the motor)
the pto "clutch" is just a big electromagnet, you can test it by putting 12Vdc across the two wires. With no voltage, the two parts should spin independantly, with voltage applied the two parts are pulled together and will NOT spin seperatly)
It also has an adjustable gap to allow it to "slip" when non powered. I believe the correct gap is .012 you use those three spring loaded nuts to set the gap.
your dealer said it was gone? I would ask what GONE actually means?
is an actual metal part broken??, a bushing shot?, or maybe one of the windings in the magnet bad??
often a take apart and a good cleaning and re-gapping is all that is needed!
ask if more questions-