This is an interesting question. I suppose all you need to do is multiply 7/8 X 20 mph and your answer is 17.5 mph. I don't know how that would translate on the hills. There would be more torque but would it be worth the top speed hit?
My 1.6 horsepower Tanaka motor is also a bit underpowered for my mild hills and strong winds. That's why I ordered my replacement kit with the one inch roller. I've estimated my top speed at about 20 mph. When I had a speedometer on the bike I knew how 27 mph was with my Golden Eagle kit and did experience some slower times. Mostly I ride at maximum speed.
I also want to know about 49 cc motors that would bolt directly to the Staton friction drive kit. The Honda costs plenty of money. The Honda name has a good reputation. I've owned other types of Honda engines and haven't ever had a problem with them. I have no idea about the Titan brand. It would be good if there were an independent user who could testify about their longevity.