Could you even get it?
I am not that individual and have no knowledge of the situation you are referencing.
Liability insurance would answer litigation issues and take responsibility in situations like the "fellow" looking for a "builder". Situations like that are why it is wise to have liability insurance. This topic is no different than a automotive garage, any other business, having liability insurance for work they do.
Seems liability insurance is a touchy subject.
I am not a builder and won't even do simple repairs for people either, and have been asked to before. Not on mowers saws trimmers anything..I don't want the hassle of something down the road biting me for a few bucks. And I do get asked, twice in the last week actually. I did it with cars years ago, until the day some lady was gonna sue me because her car didn't run right..and all I did was a four wheel brake job....nothing to do with the engine, but it not working was "my fault".. Made me think about stuff like that a lot, and just quit doing it. Oh, I finally convinced her brakes had zip to do with the engine, but she initially was rather irate and threatening....
If I went as a legit business following all the laws, etc, sure, including the insurance, but not on the side, ain't worth it to me. I would *sell* a used article, but won't work on someone else's equipment (outside of my job on the farm).
With that said, I don't know if you could even get liability insurance for violating epa regs and modding a saw new enough to require them.
I don't know enough of that particular law, would it be considered manufacturing? The big saw companies have to jump through some serious hoops to sell two stroke equipment today.
Say like a car dealer, could they get liability insurance for removing-modding-the cat on cars that are required to be there? I've seen a lot of signs in garages along the lines of "NO, we don't install "test pipes".