I am now pondering the idea of creating my own digital pot with discrete components that will control the darlington transitor. Any advise???

Scott,

your postings are confusing me. If you proceed with PWM you don't need a digital pot, and if you proceed with OPA544 you won't need the Darlington. So, how have you solved your problem? Do you use another high supply voltage OPamp?

Also, what you want to do isn't so simple like you might think. Your schematic published here was completely wrong! Feedback resistors were unrealistically high (100MOhm), no snubber, no Zobel network, no over current protection, no free wheeling diodes, no EMI filter, no power supply decoupling, and and and.
Driving a motor by an OPamp, even (or especially!!) when using a Darlington, is in any case a complicated task. Motor winding presents a complex load which is the best way for causing instability! So, why do you think people of Burr-Brown are adding Zobel network? Why do you think there's a short current protection? Why do you think they added free wheeling diodes? Why do you think they use massive power supply decoupling?
What will your circuit do, if motor is blocked up for any reason?

Another point: You told that your motor shall be controlled by a voltage from 0V to 50VDC. Well, there's no motor I know which will start at 1VDC, when fabricated to withstand 50VDC!! At lower voltages than about one half of maximum voltage (depends on type, of course) you will get biggest problems, when trying to start your motor. Also, there's no linear relationship between voltage and grip. So, whatever you want to do, it seems to me, that you don't have enough understanding of functionality of motor and it's driver.

My advice: Stop thinking about realization of digital pot by discretes (which is an absurd idea anyway), but try to find out whether your DC motor with it's limited range of operating voltage will do the job at all. When you must apply very low power to motor, then only a PWM methode will satisfy. With a pure DC control your motor will refuse doing anything proper....

Kai