a non self tailer cannot be converted to an actual self tailing winch... too many of the parts are different so that to convert would be more expensive than purchasing an actual self tailer...
by you saing that you are having a "challenge locating" a used self tailer, i would assume this to mean you are having a challenge locating one at an affordable price. you arent alone in that.
there are a lot of good used self tailers around... you can go into almost any marine exchange (used boat parts) and see them setting on the shelf.. and some places have new ones that have been setting there for so long that they are willing to sell them at nearly the same price as the good used ones...
another place to check is boat repair yards... sometimes they have something that may work, or know of an old boat that is being stripped/parted out.