Looking at the piston wont do you much good. Taking it apart is a last resort thing if you can't tell with the engine on the vehicle.
Clean the area around the glow plug thoroughly with a q-tip with alchohol or spray nitro cleaner down there with the plug in. Once clean, pull the plug out and rotate the crank shaft via the flywheel by hand when the engine is cold. You should feel some resistance as the piston gets close to the top. On a new engine, you probably can't turn it over with your finger. On a used engine, if you don't feel any resistance, it's probably shot. This resistance is called piston/sleeve pinch. The inner diameter of the sleeve is actually smaller at the top than it is at the bottom. As the piston rises through the sleeve, it gets wedged and causes binding. This is how nitro engines are built to create an appropriate seal for compression.
No pinch = no seal = no compression = dieing/dead engine.
You can send your piston and sleeve somewhere to get them re-pinched for a smaller fee that probably what it costs new. From what I've read, they tend to last longer after being pinched again than they did when they were new. Check ebay for sleeve pinching.
Or, you may have just developed an air leak somwhere that is messing up the works. I'd first check for pinch, if there is any, pull off the carb and put some hightemp sensor safe automotive silicone around the neck (after cleaning it with alchohol and q-tips of course), put it back on and let it cure overnight. Also, pull the backplate off the engine and do the same to it where it seals to the crank case.
Also, you can first try leaning out the low speed needle. It may just be running to rich for the weather your driving in.
