It could be a faulty electric choke or sometimes called a bystarter.
My other guess would be the valves might need setting.
They may have too little clearance causing the bike
to lose compression when hot. They must be adjusted
to .002" clearance when the engine is on it's compression stroke.
The electric choke is attached to the carburetor
and has wires coming out of it.
There should be some voltage there at least when
running. The choke has a small needle that
extends to prevent excess fuel when the engine is warm.
If there is no voltage or the needle does not
extend very slightly then it needs replacing.
Let me know if you need more info.