My son brought home a 2003 yz 450f. The motor was locked up! We pulled the valve cover to find a broken valve retainer. The valve dropped and would not allow the piston to come up. The good thing is the bottom end seems to be fine. But the head is beat up. My son doesn't have a lot of money to replace the head with a new one. so we are looking for a used one. My question is what other model's or year's 450 heads are interchangeable on the 450f ? Thanks in advance Sean.