1987 XC 250 needs top end

I have a 1987 XC 250 that I just got. The bike is in good shape the ony problem is it was sitting for about 5 years and the motor was siezed up. The guy i got it from tried to unseize it but only made it worse. Now i need to find top end parts. When a friend of mine took it apart for me he found out it had a honda odessy piston and a spacer plate under the head. Where can i get engine specs to check if the cylinder, head, rod and porting are correct. Or new parts. Or also willing to sell or trade. Thanks Robert ps I live near Detroit