Its worth saying that this is very possible, although extremely cost prohibitive. The body and chassis of coupes and roadsters are usually much different.

Makes much more sense just to buy a roadster. Although a targa top or better yet a t-top could be possible for much less money.

I've seen custom t-tops done and without brainstorming too much here, you could do it for under 10k, probably for under 1k if you knew how to do body work and weld.

My hypothetical way to do this, A decent welding shop could reinforce the chassis. You'd then need some sort of support rear roof section of the car as it would lose a lot of rigidity if spaces were cut from the roof for a targa or a t-top (I imagine the best solution would involve some sort of rollbar setup being adapted to reinforce the roof, it wouldn't be pretty though). The removed roof piece(s) would need closed and sealed with weather stripping being installed as well. finally you would need the pieces mounted in place while front and rear latches were fabricated and installed.