Dan - we have a 75 22' Argy - if your fan/vent is like ours, the corners are rounded.
The fantastic fans, as well as the Max fans, fit a 14" SQUARE opening. We had a 74 31', which had the same size/type vents. When we installed the fantastic fans, the original vent opening had to be enlarged by about 1" and the corners had to be cut to 90 degrees. (and you have to clean off all the old sealant, etc.) (we had the fantastic fan guys install at a rally)
So, for these types of fans, it's not a straight de-install, re-install. (is anything on a vintage unit???)
However, Airstream, in the newer trailers (last 10 years) still install a smaller (13.5" across), rounded corner vent with the same type of small fan (there's one in the bathroom of our 96 34') This type of vent/fan is still available - it has a plastic dome & not the skookum quality of the old units.
So, buying the $70 fan/motor replacement may be the path of least resistance . . .