Leave factory under coating alone or replace it ?

As stated the oem undercoat isk tar base. The engine compartment should not have undercoat.

I would pick a spot that is easy to reach under the car, like under the front seat. Take a flat scraper and a heat gun and scrape off a small bit.

Then take torch and set small bit you scraped off on fire. If it smells like tar roofing it is or.

It might have some non oem ontop of it.

In the engine compartment you are going to have a tough time cleaning it off.