Hi,

I would not do this. At the moment you have a quite solid machine with a decent servo motor.
It may be old and a bit outdated (seems to be a DC type motor - I can see a brush holder) but as long as it works I wouldn't change anything.
The chinese spindle will be no match - and a replacement motor will cost a fortune.

But most important: it is totally different technology regarding the electronics. You'll need to change anything related to motor control - and maybe even the whole cnc controller.

Christian