It should not be that difficult. Maybe not the correct starting handle?
Maybe not the correct parts when rebuilt?
Maybe not rebuilt properly (apart from piston type/size).
First thing I would recommend is to remove spark plugs and check the torque required to turn it over. If excessive, watch out for tight bearings, wrong piston ring end gaps, or some other problem. If there is a problem, the motor may not last long before serious damage results.