How likely is it for a 42A to be promoted to MSG/1SG?

SGT(P) (Join to see) there are too many variables to really give you the correct answer. The one I would say, is depends mostly on you. You must continually separate yourself from your peers. It sounds cliché but it is the truth. Also, look at DA PAM 600-25, ensure you have what is recommended for the current grade in your MOS. Next, be successful in everything you do. Check Board AARS and statistics often. No one can predict what selection rates will be in 10 years. What about OCS, or WOCS? Good luck in the future, I hope you reach your goals and you find the answer you seek.

To give you an idea, on the February 2018 SNCO promo list, there were 16 42As promoted to SFC, 2 to MSG, and 6 to SGM. So, I would say the chances are decent. All just depends on how many eligible Soldiers compared to slots and how on top of your required PME you are.

There are not a lot of billets, so I guess the question would be better framed as "how does it compare to other MOSs?"
I can say that historically, 42A has had a bottleneck to E-5 and -6 due to a 798 cut off score. I have always been bewildered by that, as I have yet to meet a 42A that shoots 40/40 or maxes PT... much less both. But I digress.
The road to -7 is much easier, but at -8 the limitation is billets.
On a whole, comparable to other Combat Service Support MOSs, but very competitive at middle ranks.