Not seen many upgrades on here to BF20 mill but I'd suspect if your wanting higher speeds for aluminium then you'll also need to upgrade the Spindle bearings.

No point fitting high speed motor if it knackers the spindle in short amount of time.!! End of the day these machines where never meant for high speed milling so you have to either accept the spindle limits or payup and do it properly.!!

Maybe consider a Sub spindle and make some adaption for say 2.2Kw water cooled spindle you can bolt on.?