Some of the die cast may be candidates for a Helix Humper, almost no effort at all to change these out. Check out the website.
http://www.alliancelocomotiveproducts.com/
Just scroll down and click on products and instruction sheets to see if what you need is available.
Regarding other older open frame motors, sometimes it is easier and better to keep them, due to design considerations and space configuration. If you want to keep and open frame motor, but want it to draw less current and work much better, replace the magnet with neodymium magnets. If you are interested in that route, do a google search on that and you will get lots of hits with some pretty good examples.
Now, about that Bachmann 44 tonner with two motors....hmm, hmmm, hmmmmmm. No easy or quick answer on that one. I have one and yes, I did install a decoder on the two motor job. The problem is these motors are not the best and there is no way to change them out. Mine worked for a couple of years and then the magic smoke came out of the decoder and it did not run. I fiddled with the motors and found a couple of things about these. The commutator collects graphite quickly and gets a high resistance short, which makes the decoder go poof. You can religiously clean out the commutators once every couple of months and that may work. Bachmann does sell these motors at a reasonable price, but I am not sure if they are any better? What I did was ordered the newer chassis with a single motor from Bachmann for about $35 and installed it in my 44 tonner body and installed a decoder, it works just fine. The other major problem with these 44 tonners is that the gears are very poor and split and slip and make lots of noise. NWSL does sell new gears and I should buy some so I can be prepared to replace them when my new chassis splits its gears. If you want to give it a go, do a google search on dcc for two motor 44 tonner and you should be able to find some very good instructions. Basically, you need to cut off the strap that goes from the brushes to the side frame to isolate the motor. You then solder a wire directly to the brush on each motor.
Good luck and regards, Vic Bitleris