When you are trying to start the engine, do you have the clutch released, lever not locked and all the way out, and then try to pedal from a dead stop?
If so then this is part of the problem.
These bikes are best started on pavement, not dirt. Dirt is possible, but the rear tire tends to skid easier.
The following is how I start difficult bikes, you can use it to test for your spark as well if you:
1. Remove the spark plug from the cylinder.
2. Keep it plugged into the spark plug cap coming from the CDI unit.
3. Lay the spark plug on it's side, on the top of the cylinder head fins, making sure the metal of the spark plug is touching the metal cylinder head,with the spark gap facing where you can see it as you ride.
4. Use tape, a bungee cord or whatever to hold the spark plug securely to the top of the cylinder head so it will not wiggle around and fall off. Just be sure you can see the spark gap when you are pedaling down the road.
From a dead stop:
You need to squeeze the clutch lever in, do not lock it in, just hold it squeezed in.
Start pedaling the bike up to a brisk speed, at least 7MPH, all the while holding the clutch squeezed in. Once up to speed, stand up on the pedals, and quickly sit down on the seat, this is to afford the rear tire a brief improvement in grip against the ground.
Keep pedaling the whole time, do not coast at all.
Time your releasing of the the clutch lever to engage the clutch at the precise moment your bottom hits the seat, (this may take practice) and do not release the clutch slowly. Release it quickly, letting it snap out to the extended position.
If the clutch is not slipping, the engine will spin and you should see the spark plug sparking.
If the spark plug were installed in the engine, and you have no carburetor issues, or electrical issues, the engine will start with a little choke applied with the choke lever on the carburetor. (Only choke a cold engine, hot engines need no choke)
Try this and please get back to us with the results.
