I think you'll find that a PPL holder can carry out a 50 hour check on his/her aircraft with very limited defect rectification. Other maintenance checks have to be carried out by an approved maintenance organisation and signed off by appropriately licenced engineers.
Any unlicenced mechanic is 'qualified' to work on aircraft; there's no exam although some companies may ask for evidence of apprenticeships or courses before employment. The bits of paper that count are the Part-66 Licence and its various categories.