Can I install P-94 pickups on my Epiphone Riviera?

Not without routing the body. All of the P94's I've seen were humbucker sized. Usually Epiphone P90's sound good, but if you really want to upgrade get some Lindy Fralin or Lollar P90's. I have Fralin's in a custom built guitar and they blow away the Gibson P90's in my Custom Shop Les Paul.