Do motorcycle head gaskets burn ?

Hello, First of all I have a need to explain myself clearly, so excuse me if it turn out too long. I have a Suzuki GSX-R 600 1999, but they changed the engine to a 750. But as I understood, the engine is a 750cc but the head is from a 600cc engine. I understood that it gives more power to the engine. ( Don't know) Apperenlty the 750 engine is 1997 or 1998 ( Not sure either)Anyway, when I went to change the oil, I noticed it was milky kinda green oil. They told me that there's a water leakage into the motor oil. I went to a mechanic or technician, however they are called....he told me that it could be either the waterpump or water jackets. I asked him if that the cylinder head gasket could have burned ( Like in cars)...he said no, that head gaskets of motorcycles do not burn.....He check the waterpomp, he changed the seals ( but he didn't put new seals.) He also checked to head and the block, he said that everything is ok. He told me to test it, and if it keeps leaking water into to oil, to bring it back. I test it, but the problem wasn'fixed. I went to another mechanic and he told me that it can be waterpump, or head gasket. I told him, that I've been told that head gaskets do not burn. HE said that they lied. They do burn. I left my motorcycle there, they changed to waterpump seals for a new one, and now they are telling me that my head gasket is burned. Apparently they found water in one of the cylinders. ( That sounds weird to me, because if there was water there, the engine should fail. And I didn't noticed that) So I went to a third mechanic, and he said that it could be either waterpump or water jackets. He also told me that head gaskets in motorcycles do not burn, because they are made of stainless steel, so they do not burn. Now I'm a little bit confused.....I don't know who is telling the truth and who is lying. Can anybody help me ?Btw, thanks for reading