Nasty looking impeller, I can see you having to replace the impeller and maybe the water pump assembly, not sure how far the plastic flowed so trying to get it out of the engine block, hoses and rad may be a chore?
The impeller may have failed due to lack of coolant and the temp guage wouldn't read right if it wasn't in the coolant. If the coolant was low; the plastic may have stayed near the water pump which would make it easier to clean out.
Do a compression test first and check the cylinders with a boroscope to see if they are scored due to over heating. Looks like a big job.🤑