The route is fairly flat, Scottsman hill is the only real challenge on the long distance trip. Gosh its gotta be 2-3 miles (3-5km) and a grade of 2%. Its the kinda grade where semi trucks gear down and steadily make their way up in the left lane.
If I go with the smallest gear on the 30cc trimmer motor shaft then a huge tooth on the wheel, say #35 - 72T, or go jack shaft for higher gear ratio would be enough?
I double checked my speed requirements and 32kph(20mph) is about right for my needs.
If I buy the $20 trimmer 30cc, make it work, and its not powerful enough, then I hillbilly up a second $20 gas trimmer motor to get that extra bit of power.
You might try zenoah g43l from Staton. Friction drive though. But I don't know about 425lbs. I have one with an upraded carb and have been clocked at 40. Check it out on YouTube. search "friction drive motorized bicycle by Bishop". Best two stroke I have ever owned.
I have a 21cc Echo FD powered bike that easily moves 200+lbs of bike and rider at 20+mph. However I live in the coastal flats with no hills and that's about half the weight you want to move. Get something bigger.