The 280 Volvo sterndrive is a reliable performer. It is not designed for high-performance, but for reliability.
I'm going to presume that if the I6 cyl. engine is original, you have the AQ170A or AQ170B VP system. it is the 181 ci GM block, which came in 170, and 160 hp versions. 170 hp was the most common, I think. The engine is pretty strong, and infinitely rebuildable. It is not, however, very economical. It's a good motor, just not a great motor.
Conversion to a Mercury inline 4 cyl. is pretty easy from a Volvo AQ170 w/ inline 6.
The Volvo-Penta brochure is here:
Volvo Penta out of production
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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