For a lot of cheap bikes, when it comes to the cost of replacing a part like the fork, it often makes more sense to replace the bike. In this case, you have an 8 year old bike, which will have worn components, putting on a new fork (and low end) , for 60% of the orginal cost makes little sense, when for a little more you can have a complete new bike. Added to this you have fitting costs, work out how much it will really cost vs a new bike, or even nearly new of c-list of ebay