I am not sure that is the case, as far as I am aware the only difference in maintanance between private and public is the items that gone be done by the owner and the allowing of an engine to run past life on condition.
As far as fitting different parts it is not like a permit aircraft where you can pretty much fit what you want. I stand to be corrected but I was under the impression all parts had to be approved for a CofA aircraft regardless of public or private cat.