My milling machine started life as a standard Sieg X3.
There was a company called Promica that did a conversion kit as well as sold these as a turn-key unit, which is what I purchased.
There are quite a few videos on YouTube showing their equipment in action.
Unfortunately they subsequently went out of business and I am now on my own! The controller is propriety so when this fails, it will mean a total upgrade, probably to a Gecko G540 like my mini lathe. The machine came with a 4000rpm conversion, but the fastest I can get is about 3400rpm.