You need a front sprocket of 24 and a rear one of 16, something like that, which is very doable on a BMX. It won't give you much clearance.
Then, you need to get a sick brake. If it has a U-brake mount, getting some of the clear inspired v-brake pads will do, and you need a good lever, for both brakes.
Finally, you'd need a stem for a mod bike, and handlebars as well.
And at the end, you've got a bike that doesn't ride very well for trials.