If your engine used to start and then when you came back to it one day it would not it could be that the engine needs cleaning or it could be that your glow plug has oxidised and needs replacing. Do you know anyone else with a model so you could try their glow plug because you would not want to clean all the engine to find the problem is with the plug after spending time doing it. If you were to clean the engine though you would just use a cloth and use fuel instead of water as the actual cleaner. Usually if this is the cause the fuel left in the engine goes brown and needs to be flushed out. You can sometimes get away with puring fuel into the engine, pulling the starter a few times then turning the engine over and pulling the starter a few more times so it spits it out.