Light Aircraft Component Maintenance
WARNING: I only have first-hand knowledge of maintaining large aircraft.
From what I can see (Google), it is dependent on: type / size of aircraft and if it is operating on an EASA C of A, National C of A or a Permit to Fly.
Less than 300kgs has different rules again.
Please bear in mind that removing and refitting a component from an aircraft is considered aircraft maintenance, whereas testing on a bench is considered component maintenance – different approvals may apply.
I would start with the CAA website – General Aviation, take a look at EASA Part M, parts E and F as well as the Light Aircraft Association.
The EASA appendix II document referred to on the CAA website as listing EASA types, is no longer produced as there is a definitive list of EASA types on the EASA website.
IIRC, the European regulations are published by the commission and hosted on the euro-lex website, with links to them from the EASA website. These are European law.
The Acceptable Means of Compliance (AMC) and Guidance Material (GM) are published by EASA and available on their website directly, which they refer to as soft law.
I’m sure someone with appropriate knowledge and experience will be along soon to answer the question more succinctly.