Not sure why you would call this a "plastic engine". Is that because of all the dress up plastics around the engine?
Sorry, this looks like it would cost more to save than a new one likely cost. This all looks like damage from dust ingestion and overheating, especially the one valve that is indigo blue on the stem near the top.
Are you a hobby DIY repair type?
If so, Parts tree should have everything you need for this engine.
You need to look at the side of the engine for the digital etched digits to figure out the model and type information. The chassis label information usually does not apply to the engine parts.