Ive attached a video, but you can do this mechanically by adjusting your throttle linkage(but it may not be enough). I'd suggest making a "stop" or a mechanism to physically stop the throttle from opening up on the engine end at the carburetor or throttle body. You must add something to interfere with your engine being able to "open up", I'd reduce it so that it only opens up 15%-20% which will severely limit performance.
Another option is to add an aftermarket Rev Limiter like a MSD unit, or find any rev limiter compatible with CDI ignition and a Single Cylinder, that'll work and you can limit it to whatever rpm you prefer at the turn of a dial! I had to do this to a motorcycle and a drag car and used a simple adjustable rev limiter to keep others from mulling themselves or blowing my motor up!
The engine in a 2004 JD Buck should be a Rotax 650, which is a 2 cylinder 4 stroke water cooled engine generating 62 hp. With that said, the most likely cause of oil in the air filter and leakage from the top of the motor is excess pressure in the crankcase. A cracked block is a possibility, but the most likely culprit is worn piston rings. A compression test should help narrow it down, but in either case you are looking at a complete tear-down of the engine. https://www.rotax.com/en/products/rotax-powertrains/details/rotax-650.html