Firstly, I think you should find a model that has the visor in place already in the pepfile! Then, after you have assembled it, you should resin and fiberglass as you would with any other pep, smooth it out with bondo and just make it as good as you want, need and can! This includes the pep visor, since now comes the crucial part.
Now, after all the nice work is done, you should separate the peped (now fiberglassed and bondoed) visor from the helmet base. After that is done you should build it up from underneath, with anything sturdy that you want. I would suggest either clay, or maybe some expanding foam. After that you are set to vacuum form the visor. Doing that with a clear plastic and then painting it will yield you the best result, as far that I know of.
Another method that I've seen being pulled of really nicely is using multiple bike-visors, such as the ones used in Chief helmets, but these are layered and attached in a way that it would fill the desired space on the helmet. A bit more trickier to pull of nicely, but sure it could turn out really good too!
But vacuum forming is you best, fastest and probably easiest bet!
Edit: Seems like Carpathia managed to post first.
