After a long time, I've found a motor that suits my needs for my MAXNC15 mill. It's originally sold as wood router tool but it's the only model I've ever seen that has the motor indipendent from the router  (with a standard 43 mm collar support).
This router is produced by AEG (an Atlas Copco trademark from Germany) and the model is the 710 OFE.



The main features of this motor are:

1. Power: 710 Watt.
2. Weight: less than the original Daytona motor
3. RPM: electronically controlled from 11,000 RPM to 30,000 RPM (with torque controller)

I think this is one of the best solution.
I made a new motor support that is robust enough and simple and fast to regulate.
I also made a new pulley to get better ratio, so that I get a total range from 2,000 to 20,000 RPM.



Yes, with this i cannot regulate the spindle speed from software, but for me this is not a problem: I can adjust the spindle velocity during the job without stopping the program instead.