There were many "coach builders" that did convertible conversions on various makes and models, where the original manufacturer didn't offer a convertible. Many coach builders offered convertible conversions for 1979-1982 Capri and Mustang.
I wasn't aware that there were coach builders that did convertible conversions prior to the introduction of the "Factory" convertible in 1983. Even then the conversion of the 83's was also done by a coach builder. The cars came off the line as a Notchback with the frame reinforcements required by the road safety departments in the US and Canada and then sent to the Coach Builder.
In my personal efforts to find some non Ford parts unique to the 83 I found out the Coach Builder that did the conversions for Ford is no longer in business. So I'm SOL on that front.
With respect to interior & exterior parts for your vehicle from the back of the front seats forward including door panels, parts for the roof mechanism and maybe the Back seat are readily available from parts suppliers like LMR & CJPony. I suspect the rear side panels, arm rests and other parts related to the back seat interior trim may be difficult if not impossible to find even for the 83. Maybe these panels and other trim parts from maybe an 84 the first year I believe standardized Ford parts were used in the convertibles may be easier to find and modify to fit to your unique vehicle
Good luck