What parts do I need to make my 1996 yz125 into an fmx bike?

you need high bend handlebars, suspension, and cutouts on your side panels. those are the white (stock) covers on the sides of your air filter. then fabricate bars that you can hold or just hold the seat. in this case, you should shave down the foam seat and replace the cover. i would also recommend cutting down the back fender so that it is shorter and does not get in the way of any tricks.