I had one replaced a while back on a 225 Opti. My guess is that you are going to be better off buying a "newer" used one. By the time you buy the head from Mercury, get it here, then pay a mechanic's time to tear it apart and rebuild it, you can get a whole used one that is probably newer. It is very time consuming and costly to pull the acillary pumps, hoses etc off the old block and grind off head gaskets. Buying the used on is a bolt it on and go and you have your old one for spare parts and lower unit.
I just bought a 2004 225 Opti with 400 hours for $4200. I think just your powerhead will be about $3200 from Mercury.
That being said, either way you go, contact Bowden Marine in Humble/Atascocita 281.441.7773. Great guys and quality work.