In General yes, an Mth engine with the original proto-sounds system can be upgraded.
Cost for the basic kit is about $160 to $180. You can go it alone and get help from the kind folks here on the forum or you can pay someone else to do it for you. Lots of forum sponsors and members do this work. The rates may vary depending on what you want done as far as lighting effects or other details.
It would help to post the actual item number. Since you are talking about a dual motor steam engine I am not sure exactly which kit you need. Although you could always just call the folks at MTH, with the item number, and I am sure they could steer you in the right direction.
(Note - no need for two separate postings. Your question will get seen here in the dcs forum)
Good luck,
John Z.