Motorcycle Questions? Ask a Mechanic.
Hi and thank you for your question.
If you changed the head gasket and the engine won't turn over now, most likely the cam timing is off. One thing you don't want to do right now is force it to turn over. You could very well bend valves if you try to force it.
I do wonder if you have tried to turn the crank backwards to see if it turns like 3/4 turn then stops, in both directions.
If that is the case, I would suggest removing the spark plugs and the cams and verify the engine still turns over without the cams. If it still doesn't turn over, something got dropped into the cylinder and is stopping the piston from cresting TDC because it is caught between the top of the piston and the combustion chamber.
Let me know how that goes.
Super. Since you are going to take them out anyhow, make sure you hold tension on the cam chain and try turning the engine over with the cams out. There is another issue also which is making sure the cam caps are in the right location and not installed backwards. If loosening the cam cap bolts 1 turn allows the engine to turn over, there is a problem with the cam caps.