(;-) Now you have entered the CATCH22 zone. Big motors ,Big Hurry , BIG drives BIGGER price tag.
yOu can LOOK aroound BUT you have to be REAL carefully of what you buy for the older DC motors. You may require step/dir control and most older drives for the BIG motors were analog. AND you never know how old or used the used ones can be.
The ac drives (step/Direction)I use here NEW are about $800 each. The Mazaks here are $1800 ea refurbished.
AN option may be to replace the "motors" with something MORE compatible to the lower end drives you have. That and a little gearing MAY solve your problem and allow you a lower cost replacement IF you loose a drive along the way.
Just a thought, (;-) TP