And right there you look for the datasheet and give it the specifications and it gives you the replacements ...
http://alltransistors.com/es/mosfet/crsearch.php?struct=MOSFET&polarity=N&pd=40&uds=500&ugs=0&id=12&tj=0&fr=0&cd=0&rds=0&caps=TO220SIS&page=0
I think this is yours ...
http://alltransistors.com/es/mosfet/transistor.php?transistor=18296