It is a sticky wicket getting the right look and feel for a motorized bicyle!
There are a number of different stem and headset diameters to work with.
I use shim stock cut from beverage cans for the steering parts and the seat posts, but really don't advise it. The aluminum gets beat up in use, and will disintegrate, leaving you in a questionable situation.
Among the problems we are dealing with here is seat height, in relation to the pedals. This is problematic because we don't typically pedal, except for starting, and don't really require a tall setting for our leg length.
...and this changes the relationship with the handlebar height!
Also in the mix is the distance from the seat to the bars, and width of the bars when making a turning movement.
Flipped and turned down bars may be dandy for BTR's, but are just not practical for much besides racing.
Standard bicycle bars DO seem to put the wrist in an odd position for something other than a customary bicycle pedal-pumping action. I never liked them a Lot! lol
Tall apes are dandy for a retro chopper look, but that isn't going to work for a lot of people and styles. Handlebar width is also critical, and many styles can't be trimmed and still remain useful.
There are taller goose-necks, stem and headset combinations that are short, tall, and long (forward).
Leaned, or bent back seat-posts, and forward stretched frames also affect the outcome, in addition to the 'comfort-bikes with the crank-forward 'townie' configuration.
Really difficult coming up with an end solution that is most comfortable for the builder!
Whew!
I'll be watching to see what combination you eventually come up with and how it works for you!
There's just not a method or place where you can 'try before you buy' !!!
Best
rc