Ended up being a easy fix as the old petroleum-based oiled became sticky causing the motor to almost lockup was just harder to turn the shaft, lucky bearing were still good as motor now runs silent with no vibration after repair. Just had to desolve the petroleum-based oil and inject new synthetic oil into the front bearing, going to remove and relube all parts that need to be relubed and going to installed brandnew belts, replace all capacitors and find a battery door release button.

Did install the new belts and capacitors and rebuilt transport reinstall PCB and motor, walkman works like new, also replaced the door latch and screws that was missing.

I just bought this model and it seems that it has the same problem as yours did. Any way I can tell if it's the petrol-based oil jamming the motor? I have no problem with trying to repair it myself specially because I don't know anyone who repair walkman around my area.

One of mine was sorted by letting the motor run for half an hour in a bowl filled with wd40. If the stuff gets out again then you need to lube it, like retrodos writes.

Thanks Michiel  that worked great for the motor but even though the motor runs now it's not pulling the belt at all. My inexperienced-self thinks that the problem is either the gears getting stuck just like the motor did or the belt has stretched. Does that sound right?

Probably its the belts that's not tight enough. If you're still using the original one, its definitely time to replace it

Hey! I had the same problem; frozen motor. It didn't have any movement at all. If I would try to turn it, it would actually rotate the bit in the back of the motor where all the wires are attached. Not good..

My fix was to soak the motor in a bath of isopropyl alcohol (99%) overnight. The next day, I could finally get some movement out of it, and after about of minute of applying force it came lose altogether and rotated freely.

After that, I applied some lubricant (sowing machine oil). I'm not convinced that is really the appropriate long-term solution; it really needs some real grease like SuperLube. But how would you get that inside the motor? The lubricant flows freely and thus can easily penetrate the motor. But for the grease, you'd have to open it up and directly apply it. 

Anyways, it's quiet as a mouse when it operates, so for now it seems to have done the job.