I'm betting you really don't want that weight
on the roof of a 1979 Scamp! In those days, Scamp
didn't reinforce the roof for A/C unless ordered with A/C from the factory.
So, if you don't have factory roof A/C don't consider it, unless you want a giant sunroof and floor A/C when it crashes down.
But if you do have factory A/C maybe you're thinking of a heat strip rather than a heat pump?