Colin,
After taking off the door card, I believe you need to unbolt the whole mechanism and remove it as one....I would be surprised if you could get a replacement lifting arm itself, but once you have removed it, maybe you can then straighten it. I also have lost the door glass off the lifter...the glue seems to disintergrate over the years...but I cleaned out the runner, filled it with silicone, replaced the glass, raised the window and left it to dry...that was 3 years ago and all is still well...I know I shouldn't say that...
Should you need to replace the whole mechanism, I would suggest you contact someone like Douglas Valley Breakers, who provide a very good mail order system on second hand parts, and guarantee that each part is in working order.
I am under the impression that the mechanism is the same for a targa,coupe,convertible etc...
Hope this is of help to you.