Another option is to give the folk hero a fictional young companion who could serve as the MC. This was done in a recent (eh, '90s) novel about Robert the Bruce (whose title I can't recall at the moment, but I look at it every time I'm in Half Price Books), as well as Gloria Skurzinsky's SPIDER'S VOICE about Abelard and Heloise.
The thing about doing that, however, is that you must find a way to make the young companion the MC--so the central story problem must be the MC's, not the folk hero's, etc.