There is every chance that it would not pass an MOT with a rusty wheel arch. Particularly as it's a cabrio and the strength lost by removing the roof is fed down the windscreen pillars and into a strengthened floor pan. The best way to do it is to cut out the rot and have a repair panel welded in.
Fibreglass or pop-rivets are a bodge, and do not restore any structure to the car! If it was the front arch, it would be easy enough to change the wing, but at the rear your options are limited.
Also check that corrosion hasn't spread to the suspension turrets, or near any seat belt mountings? If it has you are looking at very costly repairs if you can't tackle the job yourself!