Repower with the same thing, unless money is not a consideration. Modern GM 5.7L small blocks can easily make 300-350 hp without undue stress on the motor. Most old ones are 260-270 hp, but that's 40 year-old technology.
You can get a crate long-block motor 350/300 hp for $2300 from
Advanced Marine
I think a base crate motor 350/310 hp is about $4.2K and a drop-in complete crate motor for a 350/310 hp is about $5.2K
__________________
Common Sense is learning from your mistakes. Wisdom is learning from the other guy's mistakes.
Fr. Frank says:
Jesus liked fishing, too. He even walked on water to get to the boat!
Currently without a SeaCraft 
(2) Pompano 12' fishing kayaks
'73 Cobia 18' prototype "Casting Skiff", 70hp Mercury