ORIGINAL: i live for 3D
is that a sign the engine is gone?
Nope, it's just a sign it's time to clean out the carbon
The quick and easy way is to just put the piston to TDC (if it's still got a good pinch then make sure it doesn't lift the liner) and start scraping off the carbon. Usually most of it will just flake off with a fingernail or use a wood or plastic scraper. It's not necessary to get every tiny bit off, just get it dwon to where the crown is flat but probably still stained brown. When you're happy with how it looks then very thoroughly wash away any particles before you move the piston down, then wash again because some particles will have stuck to the liner. Clean the cylinder head as well. Carbon on top of a piston is no indication of what the engine is like, all it really does is bump up the compression a bit.
The photo below shows one of my engines after about 30 hours of running on 20% castor. It was still running like new but looking a bit filthy.