Indeed an inspectors signature may be required for the AIRCRAFT (the stress on the word is deliberate) following the work, depending on the work carried out, but that wasn't the question asked. The question asked questioned the need for the person doing the work on engines to hold approval, which I answered absolutely correctly.
I did point out that the logbook would need an entry, but assumed that the questioner would know about counter signatures for certain types of maintenance on microlights.
Microlight engines themselves are not subject to certification or control, irrespective of the association that happens to look after them. Many were originally converted snomobile, jetski, motorcycle or other non-approved powerplants, so by definition are not exactly subject to conformity processes in any meaningful way.
BCAR Section S, the airworthiness standard for microlights, says little more about powerplants other than they shall be demonstrated to be fit for purpose.