Help with bike recommendations

Is the frame steel?

Mike, IMO you are better off with a steel frame than AL because IF there is a problem with cracking on down the road, it's easier to find someone with a stick welder or wire feed than someone accomplished in TIG welding.

There's a lot of vibration present when going to motor power, as opposed to pumping a bicycle... and more speed overall.

Gears?
What for?

As another member stated, YOU are first and second gear, the motor can take over as third/hi-gear when you release the clutch.

IMO, you would do well investigating bikes with both front and rear brakes, disk being the best available, today.

Under power, a sole coaster brake is an invitation to disaster.
No one knows when a failure will occur.
and that will be a surprise

Best
rc

There are a few Micargi Falcon builds on this site. I have heard no complaints about them.
A 2 stroke motor fits in the frame, but you may need a modified intake for carb clearance.