I think the first thing you want to establish is whether the starter is not functioning or whether the engine is locked up. If you put a wrench on the crankshaft pulley (it's a 27mm on mine) and set your socket to go clockwise, you should be able to turn the engine over with about as much effort as pull-starting a lawnmower. Make sure the fuel is off! If it turns, something is up with the starter. If it doesn't, something is keeping the engine from turning. I'd look at the hydraulic pump, it's possible the starter is jammed in the flywheel or there is internal damage.
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