lawn tractor not charging

AFAIK the Cubs have an automatic decompressor bolted to the camshaft, and yes, they do fail. The test for this is a compression test. If it's working, the compression tester will read usually, around 60psi. If it's broke, the pressure may be over 100psi, and like bschief says, the starter may not be able to crank thru the compression stroke. But it will try and the compression will stall the starter and when you release the key, the compression will kick the flywheel in reverse pretty hard.
If the starter cranks thru at least on compression cycle, I would suppose that the decompressor is fine.
To replace the decompressor on a flathead, the engine has to be completely torn down, and usually, the cam has to be replaced, and the valve lash adjusted.