Coupler and wheel question

[ThoroughBreed]

Actually, the trucks hitting the coupler is fine. If your turns are so sharp that they (trucks) hit the coupler boxes, then a truck mounted coupler would definitely hold up better and less of a chance for derailment. Even on 18 radius, the trucks shouldn't hit the coupler box. If you need to trim the trucks a little, then do it, or space the coupler box out a little more. (I'm thinking your trying not to trim off the existing coupler boxes here, hence my suggestion)
If you decide to use body mounted boxes, you must remove the truck mounted boxes for clearance anyways. The best thing to do then, is to trim it off the truck at the point where it starts to turn into a coupler pocket. Body mounted couplers provide more stability, especially when the cars are being pushed.
You say the broken truck is a Roco, then it should be easy to find a replacement from a hobby supplier, or as suggested, fill the hole and get a pair of replacement trucks, tap and drill, and use them.

 

[UP2CSX]

Actually the centering spring is installed on top of the coupler, so he does have them installed correctly.
http://www.kadee.com/html/5ins.pdf

Ooops, you're right but that centering spring is still sticking out way too far. It looks like the coupler box is too short. It seems like it would be better to trim off all those truck mounted coupler boxes and then use Kadee boxes mounted directly to the underbody. The ears can be trimmed off to give come more clearance but I'd have to see the truck without the current coupler pockets to guess if the would be a problem with obstructing the trucks on curves. In theory, they shouldn't be. I've converted a bunch of Roco/AHM cars and they've never given me problems as long as I had the Kadee coupler pocket mounted correctly.