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You're just looking to change the aspect ratio?  I don't think there's a built in way, the height/width appear to be hardcoded in include/tic80.h.  It doesn't look like it would be that hard to swap them and recompile, although I'm not sure if the UI could adapt.

Not a proper suggestion, but I remember for some crt games the suggestion was to turn the display on it's side.  A number of monitors also have stands that rotation - you'd need to