as PM'd to you the 3.5HP has only 85 cc displacement ; whereas the 4, 5, 6 have about 125cc displacement. So, just by displacement comparisons you'd probably only max out at slightly under 4HP.

The real key here is to measure the rpm (cover off and with an optical tachometer) at full throttle at YOUR normal 'boat load' and compare to the engines 'rated' max. rpm. If not up to near max. rpm, then consider a prop alteration.
You get max. HP output at max rpm and if your current situation is not developing close to full rpm then youre not getting max. rpm.
Caution: if you exceed rated max. rpm via a prop alternation, ... wear a flack jacket and helmet.