As long at is ALL metal, cast iron, brass, and steel, (or with cast-aluminum upper housing), it will have no problem with a size 18 (and maybe) a 20 needle and Tex70 thread. Those would include most of the Kenmores though the 1970s and most of the Class 15 machines from Japan. Also, many Singer home machines sold through the 1970s, as long as they were expensive then and there was nothing but metal in the drive train
The motors are 100-130 watt rated, but it is trivial to hang larger motors on many that already suspend them from brackets. I'm not supposed to be able handle bonded Tex90 thread with those, but for short duty cycles, like for polyester straps, those machines and an early Necchi, too, seem not to struggle at all. No broken needles. No overheated motors.
You refer to bring out a "portable". Well all of those machines I'm referring to were portable, in their day, with carrying cases sold for many. 30-40lb portable.
Just to be clear: I'm referring to a duty cycle that is at most what you can be done by a skilled tailor. All bets off if you expect the feed dogs to drag, unassisted, heavy canvas or dozens of yards of lighter canvas all day.
Put it another way: Look at what Sailrite, a capable company, is selling for $800- $1100. http://search.sailrite.com/category/...arm-big-n-tall
Machines with the same build strength from 1940-1975 are at your thrift store for < $50.