This is a Video Illustrating how to disassemble the top end of a GY6 150cc (157QMJ) in preparation for a BBK (155cc 58.5mm) and a 4 valve head.

I want to make a note, In the video I say you can use the same 90 link chain. You can NOT. The 4 valve head is taller than the stock head and requires a 94 link chain.So, You must either split the crankcase and completely disassemble the engine, or break the chain and add links.