As an example, we used to run a early Seneca as a taxi, back in the 70s. On a trip with px, the first indication of a problem was the front seat passenger threw up suddenly. The pilot guessed the cause and shut off the heating while diverting to the nearest airfield. He then bought the a/c back to base for us to look at it. I found a hole in the stb engine cowling in front of the intake for the heating. The exhaust had cracked and the gas had burned it's way through the GRP on it's way to the intake. Since we needed the a/c the next day, I pulled the manafold for the approved welder to fix and worked late into the night at home remoulding the cowling with a slight bump to give a bit more clearance over the pipe. I told the CAA area surveyor, but I can't remember now if it resulted in any action.
I expect modding the cowling would be a no-no now.