Is the issue with your current motor just the powerhead? They have used or even remanufactured powerhead's on eBay and elsewhere. Might consider going with a remanufactured one, run it for a season or two and then sell? Or as mentioned before, you could always fix up and sell off what you've got, and then look at another different boat that's already been repowered or has newer outboards on it?
Models/Hp: 150/200hp HPDI 2.6L. Genuine Yamaha OEM Powerhead(Long Block). This Is A Remanufactured Long Block Assembly. (OEM P/N: 67H-15100-00-1S Crankcase). Color: Grey.
www.ebay.com