Most all wiper arms are retained to the splined drive adapter by a tiny stainless clip or retainer of some sort. That clip should be "pulled back" in some manner, I use a hook scribe or a small pick of some sort to do that.
In many cases, the splined drive adapter is either aluminum or zinc, therefore after years of weather exposure, corrosion between the dissimilar metals becomes a problem. So it takes some finesse to remove the actual spring-loaded arm while releasing the retaining clip.
This pic is a new anco "classic" series adjustable arm. Those are outstanding products for use on any old iron for repairing/modernizing the wiper arm assembly.
They come in two different lengths and are telescoping to mate up with any old iron windshield/wiper sweep configuration. They come packaged with several different replacement splined shaft adapters that will work with any common wiper motor shaft diameter.
A Scout 80/800 would use the "short" version, anco p/n anc 4102. Or the "long" version can be used and the actual wiper blade end extension cut off as needed.
This arm will not work in place of the "articulated" wiper arm used on the driver side of the Scout II without extensive modification.