I originally designed it to slide into place and be held by air pressure and magnets.  There is a central screw, that on the last attempt, I added a larger diameter washer too to make sure it held to the scoop and the screw was glued in to prevent screw loss.  On my last flight of the winter season, it fell off as I was on the final turn for a low high speed pass.  The pictures below should help explain how the scoop was connected and I will show what I have done to solve the problem.

At the back you can see two holes for dowel peg aligners and the rear magnets.  In the center of the wing is the screw that holds the front of the scoop on.

Here is the slot that the center screw on the wing slides into.  

For the fix you can see in this photo that I have carved out the foam filler, added a 1/8" piece of aircraft ply, and a straw.  The added ply layer acts as a washer for a machine screw and the straw as a guide.

Here is where the straw comes out on the bottom of the scoop and I have filled in the bobo with some Evercoat body filler.

Here is the scoop in place and the straw exit area cleaned up and painted.  I don't have a picture of it, but after the matt clearcoat, it blends in perfectly.

At the point where the machine screw meets the wing.  I cut into the wing and inserted a 1/2" diameter dowel straddling the F1 wing formers.  Then I drilled and tapped the dowel for a long 4-40 machine screw.  This should now take care of this problem.

Now the P-51 is ready for the flying season...oh, but wait .  

While trying to cycle my NiMH battery packs, I kept getting an error on one of them.  I'm running dual batteries as a redundancy system, and this is a perfect example of why.  Long story short, I ordered a new battery to replace the one.  Then after an autopsy I had one bad cell in the five cell pack.  I also discovered that my wire ties holding the batteries in appeared to be damaging the outer wrap of the battery and possibly the negative wire underneath.  I went back and added some more battery foam wrapping along with protective electric tape over that before wire tying them back in with a little less overall pressure.  Now, I'm ready for flying season! 

I thought I had posted something about that previous experience here on this thread but I couldn't find it going back through the pages, so I will document and explain the issue now.  This may help someone else out in the future.  I also sent a note to HighViz RC along with pictures so they are prepared if any stock parts left over from Robart fail in the future, and so that they can beef this up when they start manufacturing it.

Pictured here is the trunnion shaft for the Robart #145-6 retracts.  You can see where the inner cam pin has been sheered off.  I am no metallurgy expert but the exposed metal at the sheer point has a crystallin structure similar to cast iron / pot metal.  What that means, I don't know, what I do know is that it failed.

My Initial thought was I will have to order these from HighViz, but after a little inspection it looked like I could grind away the welding at the back side of the pin and then punch the pin out.  Well, that worked like a champ, and the pin came right out.

Now, what to replace it with?  The pin measures exactly 1/4" and I happened to have a couple extra grade 8 bolts left over from my P-40 axle upgrade.  On the farm, if I want to do some serious bush hogging, I use grade 8 bolts as PTO sheer pins.  Man they can take some abuse.  So with that in mind, why not give this a try.  So I replaced the pin with a 1/4" grade 8 bolt cut to size.  I cleaned everything up really well with acetone and glued it in with red Loctite.

I found a few photo gems on Facebook after the fly-in.  Now I finally have a few photos of it in the air .

This photo captures the moment the right gear breaks and the gear is on its way inward.  After this the plane continued to track fairly straight for about 10 feet dragging the right wing tip then as the plane started to turn right the left gear gave out and it folded up under the plane.  Miraculously, the prop was spared.

First landing attempt went a little long so I slowly throttled up and went back around.  It made for an excellent photo .

Is it possible to replace the retracts with Sierra's without major surgery? 

I have not looked into the Sierra but I have been offered a set.  I do think it will involve some extensive surgery.  The Robart struts swing on the outside of the mechanical workings and I believe the Sierra swing in the center between the mounting holes.  Two completely different setups .  Not to mention the gear doors would have to be modified or completely made from scratch again.

I got the left wing all fixed last night but I still have the right opened up waiting on the red Loctite to cure on the trunnion.  I will take some pictures tonight and post them to show the setup.

I don't have a set of the Sierra, but I think you can tell that extensive surgery followed by cosmetic modifications would be in order.  Oh, ignore the Zero in the floor missing the firewall .  I didn't mean for that to get in there .

Here is a shot of the trunnion back in place working perfectly.

Here is the other side showing how the retract cover and strut door look.

On this phone the Zero is not noticeable but if it is in your way, I will be glad to help you with that ?? 

P-51 is back in the trailer ready to go!

The Zero is under repair and should be ready to go come your warbird fly-in on the 17th .  Unfortunate wingtip stall created by a wind gust at the same time I decided to abort and go around.  Ended up cartwheeling in the wheat.  Broke the front end off and damaged the right wing.  Looks to be a straightforward repair.