15 ft Grumman sport canoe - buying new motor

I've fished from a 60's era 15 ft Grumman sport canoe I inherited for the last 9 years. I love it. I started off with just a trolling motor. I've had several clasic 2 stroke motors the last 6 years. Most recently a 1956 Evinrude 7.5 HP. I'm ready to step up and buy a new 4 stroke motor. I'm thinking of the Mercury 4,5, or 6 HP models. The 4 HP has a built in tank which would be nice to save room. My old 7.5 pushes me around 7 MPH. I load her down pretty good at times. With 2 people and trolling motor batteries and equipment I estimate 600 to 700 lbs load. I'm hoping someone with experance with the same boat can give me an idea on the speed I can expect from a simular new 4 to 6 HP motor. I'm guessing 7.5 HP in 1956 does not equal 7.5 HP today. (I'm told the HP was measured at the head back then and at the prop today). I think safe top speed would be 12 to 15 MPH. How fast can I expect a new Mercury 4Hp or 6HP motor to push a set up like mine? I'm willing to spend for the 6HP but if I knew the 4 HP would push me 8 or 9 MPH I would go that route. The plate says I'm good for 10 HP and 1000lbs. I also had a friend tell me I'd miss a duel cylinder motor and a new single cylinder would shake and rattle. I look forward to your replys. Thanks, D.W.