Hi,
Let me try to help you, I do have service manual/ schematics copy of this unit that can be provided if that helps.
Problem seems with spindle motor drive circuit, you can test spindle motor with 3V DC as you asked what volts are needed for that, but make sure motor connections are disconnected from circuit before to apply DC 3V on motor terminals OR these volts can damage related drive circuit.
Spindle motor is not available separately, you need to buy complete disc mechanism that includes optical block along with both spindle and sled motors etc for $67 in
PartStore.com click that link to view info.
Below is an image taken from service manual to explain "complete disc mechanism".
Let me know if you need service manual copy to check/ repair spindle drive circuit.
Thanks.