Markus, I agree with you that this bike originally had the full one piece seat pan / cowling. The battery tray located forward of the seat is a dead give away that the original motor was a Briggs. Either 3,4,or 5hp. However, I am going to throw my two cents in regarding the frame and the year. I think this is a 1964 frame. In 1963 Fox had trouble with the rear corners of the plastic seat pans cracking and splitting. So in 1964 they added two L shaped support stubs welded onto the rear of their frames in order to prevent this.
You can see them in this fellows pics. The bolt head of the rear axle is another clue. Prior to 64 the rear axle used a castellated nut on the brake assembly (left) side of the bike and a castellated nut in the right side of the axle. In 64 they began using the Bolt head on the left side and a castellated nut on the right side. Lastly, the underlying red color does not appear to be the original HiFi metallic red that Fox used on some of their earlier Campuses. Of course it could be faded but it should still be a much deeper shade of red. For me that would rule out the frame being an early version of the Campus with retro fitted handlebars. I'm going to say this fellow has a 1964 (1300 series) Fox Campus with a non original seat and motor. What are your thoughts on this. Ogy
I have never seen a bolt used as a rear axle on a suspended Campus from the factory. My late one has the same axle as the early bike I had which is a stepped shafted with threaded ends on both sides, drilled and tapped on the brake side to hold the access cover in place so they could never use a castle nut/cotter pin on that side...they used a Hex nut noted as having elastic stop in the parts lists on brakeside in the early catalogs and called it a lock nut in the later cats (with the same part number).
This is what was on my straight bar Red bike.
They did go to a Bolt for a front axle in the later years rather than a Threaded on both sides axle with cotter pins and castle nuts.
Be interested to hear whats underneath the black paint, There is definitely a distinct color difference between the frame and bars, you can see the blue popping out at least on the upper section/handlbars. I do agree that whatever is on the frame is pretty dull looking. That red would fade pretty good, but you would think it would be pretty shiny on the undersides.
This one had some pretty faded/dull areas on it, This bike has the supports for the seat pan as well.