It might be something as simple as a sticky valve. Pull the valve cover to see/make sure the valves are working as planned. Press the valve stem with a screwdriver, and let it snap shut a few times. Then see if the compression improves. I did this on a motorcycle that sat for a number of years. It had leaky valves. After the snap treatment it had enough compression to start and run reasonably well. After 20 minutes it was it's old self.
On that note I watched a fellow load the cylinder with oil, install the glow plug and turn the engine over BY HAND and bump a valve closed with the impending hydro lock. It was and exhaust valve and some oil squirted out before the valve shut.
You mileage may vary considerably if you try this.