Build your own mast.
You could make your own freestanding mast out of wood, fiberglass, and epoxy. Contact Gougeon Brothers for plans on doing such.
If you put the mizzen in the mainmast step, the boat will sail downwind, and downwind only. If you step it in the mizzen step, it will round up into the wind and drift downwind backwards. There is no provision on deck for any sort of stays. The structure is not there to support them.
The only real option here is to purchase or build a proper mainmast.