Yes, Sew-Classic is the place I go to for belts. She won't have a direct listing for the SEWMOR but she does have instructions on how to find the belt size.
Basically tie a non stretching string around the motor pulley and hand wheel with the motor in the center of it's adjustment range, then remove the string and measure it. She has detailed instructions on how to do this in her site.
My SEWMOR 606 is very similar to your 754, but the belt on it has a non familiar number. If you want to email Jenny at Sew-Classic and give her the number to see if she can cross it, the belt number on my machine is 1100 L. I have no idea who made it as it was on the machine when I got it.
Joe