I've actually been thinking about this very recently for my own machine. I'm not sure whether a full-size 0-13mm chuck would fit in the opening but I haven't properly measured it. If I remember off the top of my head the standard collet holder is 35-40mm or so in diameter and there's perhaps 10mm of clearance between that and the casting.
Then, even if a chuck could fit, you'd need to either thread or put a taper onto the motor shaft to hold it which involves stripping the motor down to get the rotor out and having it professionally done unless you can do it yourself. Alternatively, you can get ER20 collet holders which hold up to 13mm rods that will fit on a motor shaft but I think the largest bore you can get is 16mm, so you'd need to have it bored out to 19mm or 3/4", whichever it is.
In all, there's no real easy way to do it from what I can see.
It's a long time since I broached this subject , but we are having another go at solving this problem , been too busy .
Did you solve this problem trevanion ?????? If so would you be so kind as to pass on your findings.
Or is there any one else who has sorted it out and would pass on their findings as well. Any help would be much appreciated. Thank you desmo11225