I have worked in many shops, as machinist and have been an engine tech, and from day one as a youngster I've seen many so called experienced in both fields that know very little and do shoddy work. I asked myself many times how did they ever get into the field. No longer a kid.
I found nothing that said an A&P has to do said work, as in many shops there are many workers with no A&P, and shops I've been in(visited) no one is standing over the techs. Also if said engine goes on an old war bird it does not have to be certified, it is done so under the aircrafts certification. None of the (edited) military class engines were ever certified.
Its up to an AP to inspect and approve said engine for use, and if its say an R-2800 that was found in a barn in Mexico and the AP gets it running and approves it then, my question is answered.
I've searched and can not find a full part 43, just some bits and pieces, revisions etc. I may have found it after this post.