Try turning the flywheel to turn over the engine. If it is still difficult to turn over (with glow-plug removed) then most likely the engine is siezed in some way. If it is much easier to turn over then the pullstart may be binding in some way, although i doubt the 1-way is the problem, as they usually just slip.

Try carefully loosening the four pullstart screws a little and try again - overtightening these screws sometimes causes the pullstart to jam.