Dennis, I had to use a 9.9 to get my 22' seafarer to home after main outboard seized. I was on long island sound by Stepping Stone lighthouse, and putted all the way to ramp on Hudson River under GW bridge. A good 15-20 miles. I surely didn't break any speed records, but I did make it, and that was through "Hells Gate", Harlem River, and East River. I would imagine that 7 1/2 would move your 24 along in fairly calm conditions, but probably just barely. I'm keeping my eyes open for a 15-20hp kicker for my 24. Hopefully I'll find a good deal on a x-long shaft 4stroke so I can run a line from the fuel separator to it, and not need an extra tank on board.
Since you have the 7 1/2 already, why not throw it on and see how it does. It may surprise you!