Congrats! I think that 305 is the MPI 260HP version which is about right for that hull. If it runs well I would not even think about a bracket&repower unless you really want to get into spending some time & $ on a rebuild. I suspect it would cost a lot more than you paid for the boat and thats doing it yourself. A new Yamaha 250 is around 20k. You would also have a bit of glasswork and the cost of a bracket. If you take good care of your I/O you should be able to run it for a lot of years. There are a couple of good threads here about that; one of them is by Pelican:
http://www.classicseacraft.com/commu...ad.php?t=23364 .