Big boy scooter - at wits' end

[Walterp79]

Please help I am desperate. I blew my gy6 engine, I think the timing was out. The piston burnt and the chain adjuster broke. So I replaced the piston, barrel, head, chain adjuster, chain guides,oil pump. It cranked without starting. Then I cleaned the carburettor. It cranked without starting. I checked the fuel flow. It spurts when I turn the petrol. The carb bowl has fuel. I checked the spark. I also did the valve clearance properly. But it has a weird plunging sound when it cranks. Any ideas?
Oh, I also opened the starter motor and cleaned it. I thought that the plunging sound could have been oil inside the starter. There was some black stuff at the bottom. Not sure if it was oil.
I have 5 bar of compression. The guys here says that's the best I can hope for it should start.