Make sure you say B+ cap, not B. There are plenty of B+ models with the cap screwed to a regular filon body like yours. The width of the body will have to match exactly. Probably the height above the cab roof will have to be quite close.
If you call Bontrager's Surplus in Michigan, just north of Elkhart, ask them if they know a source for fiberglass motorhome front caps, they might have a tip for you. If they don't, ask them who they would call to find out. If you are not shy about calling around like that, you'll find someone.
Elkhart is ground zero for RV stuff, that's well known. Anything and everything you could possibly need for an RV can be found somewhere within 30 miles of there. Including repair guys. There are small component factories all over the place. But there does not seem to be any easy way to search for any of it online.
I'm sure there is a cap made that fits your body's size, if you can manage to find it.
Refitting in this manner sounds easier than it probably is. Myself, I'd probably throw in the towel and tear it apart, and rebuild it. There are plenty of documented rebuilds on here and elsewhere on the internet. It's more time than money.
Did you ever read the Tioga and George blog? He had a fiberglass guy in Mexico take off his corner trim and simply fiberglass over the seams. Turned his rig into a poor man's one piece body. It's another outside the box idea.
