Your mill is in much better shape than mine. The fellow who sold me my mill owned it from new in 2000, but he was not one cleaning up the chips so the Y axis ways have some grooves where chips got underneath.
The seller pointed out that the mill does not have a quill lock. He said he holds the spindle pulley with one hand while using the wrench with the other and never had a problem tightening or loosening the drawbar. So far he has been correct.
I did not like the speeds on my mill. Nothing in between 490 and 950 rpm. Also changing the belts was a pain since insufficient movement on the motor mount so the belts were not really loose enough.
I also had some belt noise due to the auxiliary pulley being about 1/2in higher than the spindle pulley and no easy way to tweak the alignment.
I finally removed the auxiliary arm and pulley, and flipped the motor pulley upside down. I now only have 3 speeds, but they are ones I can use. I measured with a hand held tachometer. Speeds are now 560, 862 and 1200 rpm.
The link belt runs smooth and quiet. The long length and the links make changing belts easy. A lot more stretch than a fixed belt.
I was also able easily align the motor pulley with the spindle pulley.
By the way, if you ever need to remove the motor pulley, mine has TWO set screws in the pulley.
I want to eventually change the motor to a 3 phase motor with a VFD. Each month some other bills get priority over this purchase.
If your motor is the same as mine, it may be a non-standard motor. Mine has a 3/4in shaft and if I recall a metric sized key.
NEMA motors would be either 5/8in or 7/8in. The motor mount spacing seems like a NEMA 145 which has 7/8in shaft. If is easy to purchase a single pulley for 7/8in shaft, but not easy to find a 3 step pulley.
The previous owner said the original motor was making a noise so he replaced with a 1 HP NEMA frame 56 motor which has 5/8in shaft and made a bushing and deeper key. I took the original 1 1/2HP motor apart. Something fell out, likely the source of the noise. It started up and ran without any noise so I put this back in the mill since the original motor is quieter especially on startup.