Sorry to say I wasn't able to find any plans at all for the M-190...or any of the other Doodlebugs, for that matter...on the historical website. That isn't to say the info isn't imbedded there....I just haven't found it easily.
I did look through my own ATSF 'library', however. The only plan drawings of the M-190 I could find were in Worley's Iron Horses of the Santa Fe Trail. They're simplified classification drawings, showing major external dimensions, weights, wheel size, engine specs, etc. There are two drawings of the M-190 showing only pre- and post- update info for the same (one and only) unit. I'm guessing you're looking for more than just a summary classification drawing, though.
You might write to the society. The website...available to anyone...will list the current officers by name and their specific responsibilities. Pick one that sounds promising, send them an email or snail-mail inquiry....and be prepared to wait a loooooonnnnnngggg time, based on my own experience. I inquired about a one-off heavy duty 4-truck flatcar that the Fe had...looking for a photo for paint/lettering guidance. As it turned out they put my question into their quarterly magazine, The Warbonnet, soliciting reader help. Of course, then they had to wait until someone provided help...which was then published in a subsequent quarterly issue! My question was finally answered...18 months after asking! Of course, I was elated to get an answer at all, but it took some time. You might get much luckier!
Sorry I couldn't help more.
KD