Anything more than 4hp on an OSPREY is a waste of $$. Even 4 hp is almost more than you can really use, but 4hp is about the smallest that will have FNR and a remote tank option (why I have a 2-stroke 4hp Johnson long-shaft on my DS II, FN only though). Going to a 6 HP outboard, especially a 4-stroke means a much heavier motor than the 15'9" OSPREY was set up for. I'd be more inclined to go with a 2 HP HONDA or 2.5 HP Merc/Tohatsu/Nissan/Suzuki, even though they don't have full (FNR) gearshift or the remote tank option. If 2.5 is not offered with longshaft, then the 3.5 hp (2HP Honda does come in 20" shaft).
6 HP will more power than can be used, why have a 6 if you can't go more than 1/2 throttle with it (maybe not more than 1/3?)?
HONDA 5 hp might be "OK", but is still borderline oversized for the boat. However, it might be a good compromise....... I'd consider a 4hp first though. O'DAY suggested a 5hp or less, but those were back in 2-stroke days.
My Dad had a 3 hp on his old FALCON (the OSPREY was a Falcon clone), and the 4hp on my DS II is plenty of power, could MAYBE handle a bit more....... but don't need it (4hp creates a pretty good wake @ 6.25 knots on a DS II!).
Going from a 3.3hp to a 4hp will be more of an upgrade than you think, you will gain the remote tank, full gearshift, and most likely, better vibration damping, and long-shaft (again, reasons why I have a 4hp Johnson, not the 3hp Mercury of the same vintage, 2001 model even though I still only have FN shift).
