You are correct with this code 342.1X. Spastic hemiplesia is characterized by increase muscle spasm and tendon reflexes of the affected (paralyzed) side. The fifth digit further subclassifies whether the side affected is dominant, nondominant or unspecified.
If an individual who is right hand dominant experience a stroke that causes paralysis on the right side of his/her body, the course of rehabilitation is more affected. This fact points to the importance of accurately and completely reporting this info through codes. E.g. Acute left sided cva with infarct in hemiplegia, right side. Patient is right hand dominant; 434.91 and 342.91.
If nothing about the side is mentioned then you can code 342.10.