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Missouri | Had one for almost two decades. Had it all the way apart, twice.
Once was a broken chain in the chain case. Had to pull the hydrostat/hydraulic pump assembly out to get to that. A drive chain broke because the nuts holding the hydrostat motor to the outside of the chaincase loosed over time, and dad ran it for a long time like that. I would say this is probably an uncommon problem.
Also at the same time, had to pull the engine out to put a new ring gear on the flywheel. If the starter begins to grind on flywheel engagement, service the starter immediately. Don't continue to drive it like that for years.
FWIW, the engine was not hard to pull out. Only takes about two hours and it's out. Disconnect coolant lines, fuel line, air cleaner tube, muffler, throttle & choke linkages, unhook about a half dozen electrical plugs, take loose the four bolts at the bottom, chain it to a cherry picker, lift it 2 inches, and roll the picker out. Easiest engine I ever pulled.
There is a little (approx 6 inches long) driveshaft from the flywheel to the pump assembly. It has two u-joints, and a slide coupler onto the splined shaft of the pump. There are three grease zerks on it, and they call for grease on the 100 hour interval. If you run it for 2000 hours without greasing them, you will find the ujoints ruined when you pull out the motor. They are damned hard to find if you haven't seen them, and you cannot see them without first removing the engine or the pump assembly. To grease them, you have to lift the cab/ROPS. If you're not familiar with that process, it's easier than it sounds. Take loose the two bolts at the front, and lift it up. It's hinged on the rear, and if the gas support springs are still good, it will hold itself up. If your gas support springs are bad, replace them. Crushed under the ROPS while working on the machine ranks pretty low on the list of good ways to die. Once you have it up, you have to reach around behind the rear pump mount, in front of the the fan/fan shroud assembly and feel for the grease zerks on the shaft. If you have a small mirror you can hold back there, you may be able to see them first to know what you are reaching for.
A few years later, it began using oil. A vacuum line had rotted on the underneath side, and it was sucking in dirty air for an unknown, long period of time. Pulled the engine again. Tore it down, and the rings were so far gone, after the head was off and the balancer removed from the crankshaft, there was so little tension on the rings I could grab the crank snout by hand and spin the motor with ease. I took the engine, stripped down to the long block, to my mechanic. He measured everything, and said it just needed a honing and rings.
While that was being done, I removed the hydraulic valves assembly and took it to a bobcat dealer to have a leak sealed up. I have absolutely no idea how one could remove that valve block without first removing the engine and transmission. Just no way to get a wrench on every fitting in the middle of that frame! Anyway, I put the valve back in, got the honed/reringed engine back from my mechanic, put it back in there, and it ran great for several more years.
Attached pics show the work on the machine the first time, and the bolt that came loose and jammed up in the sprocket.
 (Bobcat, opened up.jpg)
 (The bolt that f'd the whole thing up - Copy.jpg)
 (Bobcat hole where engine should be.jpg)
Attachments
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 Bobcat, opened up.jpg (35KB - 1189 downloads)
The bolt that f'd the whole thing up - Copy.jpg (32KB - 1157 downloads)
Bobcat hole where engine should be.jpg (59KB - 1423 downloads)
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Bobcat 642-B ??
