Bike first: you will run out of gas and have to pedal home, occasionally. For this reason the bike needs gearing for flats, wind and any hills you have to climb. The bike will be carrying you and the motor rig. For this reason, a triple in the front and a big cog in the rear that is "big enough" are desirable. The motor rig has to cause minimal friction drag when pedaling.
Cruiser bikes are designed for speeds in the 6 to 8 mph range, 12 mph is fast for one. The steering geometry is made for this range and the strength of the build is designed for the stresses of this speed range. Inexpensive bikes must use inexpensive materials and fabrication, that is they are relatively weak for just riding without a motor.
Motor: Subaru/Robin engines are made in Japan and are excellent. Last time I checked, Honda engines were not made in Japan but were made with Japanese supervision. Check the weight of the engines, the 50's are probably heavier than the 35cc engines. I use a Subaru engine but a Honda is also a good choice. China Girl / Happy Times engines are a hobby, not transportation.
Drive train: friction drive is the easiest to install and live with. When the motor is not being used the roller can be moved off the tire, it is quick and easy to do. There is no friction drag when the roller is not touching the tire. I have 164 miles on my Straton friction drive, running on a Performance Gotham tire, the tire shows almost no wear from the roller. It is difficult to change rollers or roller bearings on a Straton, there are other friction drives that are easier to service.
The Golden Eagle setup uses a cog belt to drive a sheave that snaps on to the spokes of the rear wheel. This setup is probably more fuel efficient than the friction roller. When not using the engine, the belt is dismounted and tied out of the way, I think. The motors friction clutch does have drag internally, that's why it is important to remove the belt. There is far more stress on the spokes from a GEBE setup than from a friction drive. This is an important point, with respect to durability in the long run.
So, choose a speed range you want. Choose a bike intended for this speed range: geometry, materials, components and build quality. Choose reliable and trouble free or a hobby to tinker with. Choose the time needed and success level you want and then get started.
