Most of the failed plastic cams were from around 1998-99 and were replaced with metal cams as they failed. Most mowers are not run that many hours. Working on a CC tank 60 right now that the Kaw engine has been giving me fits. Finally got it narrowed down to my stock of RN14YC spark plugs is faulty since the NGK BPR4ES seems to be out of stock that I would normally use. Currently running a set of BPR5ES.
Don't forget to grease it, especially the grease fitting on the pump belt idler that is up underneath. I might add that the Hydro Gear pumps get their fluid changed and filters every year as well. I had some issues with the pumps leaking fluid from the charge pump cover. Even replacing the 'O' ring didn't cure the issue so I took the die cast covers and flattened the mounting surface with a bastard cut file and replaced them and no more fluid loss. I see HG has an upgraded cover available now but mine are staying dry and yes, I marked them as to the correct orientation. Putting them on backwards equals no movement....lol I do like the fact you can renew the fluid and filters and I have the HG change kit with the fittings. Filters are a little pricey however.
Not sure what plugs I run, I clean them and reinstall and I think maybe I've changed them one time. My Kubota dealer is also a CC servicing dealer and I get to see some of the beat on flogged mowers that come in for repair (I work for them part time). People really don't take even marginal care of them.
Did have an issue early on with the Kawasaki, it dropped part of the alternator magnet in the flywheel and ate the stator too. I did the work myself and Kawasaki covered it under warranty. Other than that, no issues other than the Kawasaki engine is a pig on gas.
I think the one thing owners don't understand and that is, besides regular maintenance, you MUST keep the valves in adjustment. Easy to adjust but if they are out of spec, engine performance goes down the dumper and they start hard and loose power. That and removing the engine shrouds and blowing out the cooling fins on the motors. They are air cooled and need good air flow to stay cool.
Good units but like anything, you have to maintain them.