Analogue hardware aspect ratio converters do (or did, at least) exist in the world of professional broadcasting. You probably don't want to know how much they cost when new! Never seen one that has RF in/out though, you'd need to use the composite video output and then a modulator.
They can be found on the secondhand market....
https://www.ebay.co.uk/itm/Miranda-A...MAAOSwI8lZ7TDI