This is a pic of the bike that your thread refers to:
I have two thoughts, both backed by a bunch of years designing and building bicycles (and I'm a mechanical engineer, if that matters)...
First, the above pictured bike is using a lowrider style of springer fork, which due to the increased cantilever distance, is placing a lot of strain on that pierced sheetmetal joint. The more cantilever, the more stress on that joint.
Secondly, that type of fork on this type of bike should, according to the laws of physics, make the bike unstable to varying degrees due to the negative trail value. A straight line drawn down thru the headtube
should intersect with the ground at a point
in front of a point on the ground that is perpendicular to the front axle. This condition is called positive trail, and results in a rideable bike. For this type of bike, a positive trail value that falls between .5" and 2" is generally the norm.
As shown in the above photo, that imaginary line down thru the headtube intersects the ground way behind the line perpendicular to the axle. This condition is known as negative trail, and usually results in poor steering geometry. In this case, the subject bike has a headtube that is too vertical, and the offset in the fork (called rake) is too great.
The example I like to give is a shopping cart wheel; it has a trail value of zero, since it's "headtube" is vertical with the ground. Try turning the shopping cart wheel so it is facing forward, then try to push it forward. The first thing it tries to do is flip around; this is due to the zero trail. If you could rotate the steerer of the shopping cart wheel back about the wheels axle, you could then orient the wheel facing forward and push it forward without if flipping around.
On the other hand, if you could rotate the steerer forward about the wheels axle, upon pusing the wheel forward, it would immediately flip around- negative trail...
Positive trail is a stabilizing factor in bicycle steering geometry, not sure I would want a motorized bike with negative trail? Over the years I have learned that people will condition themselves to ride just about anything.
Not sure about the quality of the materials being used in these types of forks? That might play a part? I have a set of Nirve spring action forks, and although they seem to be decent quality, the joint we are discussing here is exactly where my eyes gravitate to when looking for point of failure. It really should be sleeved.
Anyway, long story short, you could maybe drill/file out the hole slightly and epoxy a bushing in there?
Here's a pic illustrating the whole rake/trail concept:
And a link to trail calculator:
http://www.wisil.recumbents.com/wisil/trail.asp?calculate2=yes#Actual Trail Calculator
