But it's best if you apply the right steering geometry right away.
If you would like to make a very light bakbrommer, i would think of welding something like Irmin has made, but smaller and less compicated.
Less complicated, because the complexity makes it a bit heavy.
Irmin's bakMOTOR (it is officially a motorcycle) has a 600cc mono-cilinder if i remember correctly.
It is not that complicated in fact. you can make a nice autocad drawing. perhaps even 3D if you feel like it (just lines are well enaugh for this job)
Anyway, I wish you best with your project! Will you show pictures of what excellent madness you fabricated?

B.