don't have the specs in front of me, but the motor size and shaft is a very standard compressor/commercial/farm duty type thing. common as dirt and cheap as chips. should be an extremely easy find for a replacement.
you can go with more horsepower no problem, though you may or may not need it. you can't go much faster though, as the spindle bearings are what limit that system to a bit over 10k rpm, not the motor.
there are other options people have done in the past, especially to add speed control. using a dc motor and speed controller salvaged from a treadmill is one old trick if you are the electronics type.
just remember it adds weight to an already heavy z axis, so don't add more than you need. i actually went the other way and added a lighter motor myself, one with very little if any additional power, which acted to increase my travel speeds for machining in light materials. your usage may vary, especially if you are planning on hogging through serious materials.
if you already are looking to spring for a new motor, you might want to look into simply ordering a 0.8kw or 1.5kw chinese water cooled spindle, and locally source a small vfd to run it. they are stupidly cheap now.
this would still cost at least double or even treble what a straight replacement motor would cost you, but would give you greater power and the much higher spindle speeds you seek (24k rpm!!), as well as adding speed control in gcode, and a lighter z axis all at the same time. if you do it, better to do it now than sink more money into the spindle that is there if you think its speed or power will limit you.