I also have one. I bought new. Shear bolts are 10mm, but I use 3/8", easier to find/buy. I found mine does not like grape vines. It plugged up, so I no longer try to chip them.
I bolted 4x4s to the skids to make it sit higher for less drive shaft angle.
Had one, sold it last year. Roasting sticks is a lot less arduous than chipping them. Quicker too.
The OEM PTO shaft that came with mine has a shear bolt. It's built in to the U-joint on one end of the PTO shaft. The Op Manual talks about replacing the shear bolt. Nothing is mentioned about a slip clutch.
Hmmm... I find my burn pile ritual theraputic and quite enjoy it. I burn a couple times a year. Spring/Fall. Yes I have to orchestrate the timing with weather/dryness of pile and work around the burn ban season. But I like that freedom I still have. At my old house in the suburbs, that 'privilege' is gone.I used burn piles for years and find nothing easy about them. Gathering branches from all over the property, letting them dry a couple months while they were always in my way, waiting for conditions safe for a burn, then tending the fire all day, often stretching into two or three days. Never found that process enjoyable