Since the "head" you drill off is on the outside you are correct that the body of the rivet will be inside, however most or many come through. Such as those to the acorn nuts that hold cabinets to the wall or to attach things such as curtain rod brackets. Those you can probably just push the body through with a small punch. They go clear through so the rivet body and/or rivet body and acorn nut just drop.
There are a few places that will be buried behind ensolite, me I just sort of push them in then slip a piece of bent paper clip through the hole to nudge them to the side (laying flat so they don't stick up any more than they did before) or can sometimes just sort of push or work them sideways to lay flat under the ensolite from the inside. Only ones I can think of where this was an issue was the rain deflector over the door. Most of the rest came all the way through and held something in place.
if you pull those and re-bed in new putty the rivets are easy to flip out by slipping a putty knife behind the ensolite from the window opening after the window is removed. Those rivets are very close to the edge of the window hole so not hard to get them out.
You may find common acorn nut sizes won't match up with original rivet size. I think the original acorn nuts (in scamp
anyway) were for a #10 size or maybe #12 bolt it was like the highest # size of nut. No one typically carries that anymore. Even good old fashion hardware stores jump from a size smaller in the # sizes to the fraction of an inch sizes. My solution was to buy a size too small in threads which was the same size head as original then drill a small amount of the threads out. This allowed the original size rivet to go inside the nut, and still left some threads for the rivet to grip and pull tight. I posted what the actual size nut and drill was someplace if you find you need my sizes I can probably find it. Not 100% sure Boler
will exactly match the Scamp sizes but would not be surprised.