Skigirl,
Sorry to learn of the trouble with your boat. When replacing the motor you have three options. Purchasing a new ‘Short Block(http://en.wikipedia.org/wiki/Short_block)’ which would be a new or remanufactured block, that you would put your used(or rebuilt) heads on, this probably wouldn’t work for you given the overheat issue, since your heads probably should be replaced. The next option would be to purchase a ‘Long Block(http://en.wikipedia.org/wiki/Long_block)’ which would be a whole new or remanufactured block and head you would install your accessories(waterpump, EFI system…) onto. The third option would be an entirely new motor and accessories similar to this http://www.skidim.com/prodinfo.asp?number=1630 from PCM, which would include a new block, accessories. This would allow you to switch to a ZO compatible EFI system, but likely isn’t necessary for a repair unless you also want to upgrade. General Motors is the original manufacture for the new blocks, they are marinzed by companies like PCM, Indmar or Ilmore. Alternatively you could purchase a remanufactured block from Jasper http://www.jasperengines.com/marine-products.php or similar. It’s likely you could find local engine shops to evaluate and rebuild you existing block. Ordering a remanufactured block from Jasper or other might reduce your down time.
Good luck with the repair. Where in the country are you located?
Jamie