Horizon Distributors is the owner of Athearn's product line.
Although Athearn hasn't made anything of significance in O scale (e.g., rolling stock kits) for a very l-o-n-g time, they have continued to provide O scale trucks...Bettendorf and Archbar, plastic frames, not diecast. As Jonathan sez, they come with O2R wheels, which can be easily swapped out for O3R sets.
HOWEVER...these are not always in stock at Horizon. Horizon Dist. dealers should be able to yet order them, though, and if you're not in a hurry, they'll show up some day. That's my own experience when I worked at our LHS.
Having MTH's remaining entities counted on for these replacements is always possible...but not guaranteed, IMHO. If I were in your situation, I'd try a parallel path...find a dealer who will take the time to order Athearn's Archbar trucks. P&D Hobby in Michigan would be a good choice...and, in fact, might have some in stock!!
As for the bolster, someone who has made this conversion/modification to the 19th century cars might come forth on this thread and offer advice. Of course, there are several mfrs of O bolsters, but even they might require some modification. With a few details/dimensions documented perhaps one of the growing group of 3D printer guys might be able to help!?
FWIW...
Good luck...and have a Merry Christmas, prosperous/healthy New Year!
KD