Custom is right, its basically money.
with a small turbo you can get 200+whp. remember to keep reliability in mind though. you don't want to run a crazy amount of boost without going and and strengthening the motors internals.
to build a high hp naturally aspirated motor its gonna cost you a lot more. the Erion FX 929s were putting out a little over 180hp but thats probably way more money than you wanna spend. but remember they could also easily rebuilt it if need be.