Bill:
Yes, the single-stage VM3 motor used in the MG1, MG2 & MG3 will fit in your TriStar A101/EXL, provided you use the correct motor mount. However, I would NOT do that, as these motors are very unreliable & don't last very long. Average life expectancy is between 6 to 8 years for this motor. I would take your existing motor out, use your TriStar CXL in blower mode with a crevice tool on the end of the hose, & try blowing out the motor, see if there's excess dust in the motor. That may restore it's performance for you. If not, perhaps the Ametek motor you just installed in the CXL will also fit.
By the way, in that post, you had mentioned that you thought the Ametek motor you purchased was originally the same as used in Kenmore canisters. It is NOT the same motor. The Ametek double-stage motors used in Kenmore canisters were rated at 9.0 Amps, & will pull 90" Waterlift, not 10 amps & 100" Waterlift like your new motor is rated for. Also, while the motors in the 1980's Kenmore models were all-metal, in the 1990's they switched to having a plastic top fanplate- the one where the screwholes are. It still performs the same as the all-metal one, though.
Also, the place that sold you the motor claiming it is a Miele motor is WRONG. For that motor to work in the Miele canisters, there has to be a metal ring surrounding the fan opening for the motor seal to fit onto the motor. Unless you take the one off an old Miele motor & transplant it on, or it has this ring already, it won't work.
Best of luck repairing the TriStar EXL....Rob