[prrfan]

There is a gentleman who is a member here and also an ebay seller who makes a business out of restoring older rolling stock such as Athearn etc. and also locomotives.
His user name on both sites is ggnlars. He sells them for very reasonable prices and also sells accessories.
IIRC, he has some tutorials on doing these conversions and restorations. You may be able to find them by searching the forum or messaging him.

 

[J.Albert1949]

OP wrote in #11 above (with pic):
"Here is one style I want to replace."

I'd try either a Kadee #5 (which uses the bronze spring) or Kadee #148 (whisker).

You want to be careful with that very small screw (don't strip out the threads).

For the exposed bottom, I'm wondering if it would be possible to "trim back" the bottom plastic cover (Kadee coupler sets come with plastic draft gear boxes), to clear the axle?

As mentioned above, you could also go to "body-mounted" couplers using the full Kadee draft gear box. This assumes it won't be too much of a job trying to get it affixed to the bottom of the car underframe...

 

[CTValleyRR]

Putting a Kadee coupler on a cheap car is like putting lipstick on a pig. In the long run, you may be money ahead simply by getting rid of the cars with poor details, truck mounted hook horn couplers (or cheap plastic knuckle ones), and plastic wheels, and purchasing better quality stuff to start with. But yes, anything you do to improve the quality of your rolling stock is a good idea.

 

[Jscullans]

I’m in the process of changing out all my plastic wheel sets with metal ones which I have about 25% of my cars converted. I haven’t bought a box of wheels in a while and throw the old ones straight in the trash but I’ve had very little luck with plastic wheels. I won’t run any plastic couplers and I won’t run plastic wheel sets once I’m done switching over to metal

 

[OilValleyRy]

I save about 33% of plastic wheel sets that I swap out. They find new purpose as detail parts for MOW equipment, RIP track debris and yard edges/shops.