So, there are a number of challenges you face. Firstly, what is the reason for wanting a mizzen? It will change the sailing characteristics significantly, as well as require a lot of work.
tl;dr - this is a fair old undertaking to do for no obvious reason.
If you do decide to add one, you have 2 real options - a small, unstayed mizzen off the transom, or a a taller stayed mizzen.
The first won't require a lot of work - your existing shrouds should be fine, however you will need to work a way to mount your mizzen without conflicting with the tiller. Not a major deal, but still fiddly.
The second is likely to require at a minimum:
- A shorter main boom
- A shorter gaff
- A redesigned sail
- Redesigned tiller
- Mizzen shrouds: simplest being 1 to the main mast top, 2 alongside the existing main shroud points, and probably 2 to the transom
Then you need to look at the reinforcement you will require at these points.
Once that is complete, be aware that she will try to point a lot higher, as the shape of the hull is not optimised for 2 masts, so you are likely to use the tiller more, causing a fair amount of drag. Solutions to this can include moving the centreboard or adding extra keels.