I also always weld a 'stop' on the tail boom that hits the yaw bearing to keep the tail out of the blades - this eliminates the force on the notch itself when fully furled.  We've had those get beat up/seen them get spread open (like yours) up here before - the result is the tail hits the blades and sometimes the tail bearing cracks.  

The simplest way to beef it up is to use the same pipe sizes as the yaw.  Clumsy but very strong.  No downside that I can see (in fact I would probably use it in my plans to simplify finding all the pipes if I were to start again).

However i think there is something wrong.  The arrangement should be strong enough.  The blades may be out of balance.  Or what about the weird alternator mount.  It might be causing a resonance of some sort.  Watch the machine running and you will probably see what is causing the extreme fatigue loading you must be getting.  The big overhang will certainly put more load on the high end tail stop.

The stator will get hot at times and rubs unevenly against the all thread holding the rotors. This causes the unit to squeek and to become unbalanced in high winds.

I think the "weird" configuration may be amplifying vibration of the unit.

It's time for a total rebuild.

Instead of using pipe bearings in the tail, I'll try catagory I (1/2") tractor hitch pins. Snug steel bushings are available to convert from catagory I to cat II. A clip is also included with the pin to keep it in place. They should be available at any tractor supply store for less than $10.

 I must say the unit has taken 60+ mph gusty winds, 12 to 108 degree temperatures, blowing sand and 2 dust devils. I'm still very impressed with the extreemly ruggedness and durability of the design.