I am not sure about the retrofit, but Onan has three different inside start switch configurations for the 2800. One is just the switch. One is the switch with meter. The third is switch with meter and voltage readout. Search your model Onan - look for the right one as Onan has changed the 2800 in the past few years (There are letters after 2800 that designate the model on the correct manual.) There are three manuals that I could find with a search for mine - the operation manual, an installation manual, and a repair manual. All free from websites. You should also see accessories listed for your model on the Onan website - like these switches. Don't know if all of these are available for your year/model Onan.
