is the difficulty with pedaling a new development? If so it could indicate mechanical troubles.. like a piston that's partially seized..
Remove the spark plug and attempt to turn the engine over.. If it's still very difficult, remove the drive belt or anything else that moves along with the crankshaft and could be causing resistance..
If the piston has difficulty moving up and down although nothing is interfering with it, it might be time to pull the head and perhaps the cylinder.. and see whats up in there.