There are a number of conversion kits available to buy. When I wanted to fit one to my XJ12 mill (many, many moons ago), I imported a kit from the USA from https://littlemachineshop.com/products/product_view.php?ProductID=2560 But note that it is NOT suitable for all mini mills. I failed to read the warning and found, that when it arrived the spindle pulley had too large a diameter bore and ended up making one to fit my mill.
I have an extract from a website – again American – in the form of a pdf, which has dimensioned drawings of all the parts required to carry out the conversion. It is of course in Imperial measurements. PM me your email if you’d like a copy.
Amadeal, in the UK, also supply a conversion kit
https://www.amadeal.co.uk/cgi-bin/sh000001.pl?WD=conversion% but20belt&PN=Belt%2DDrive%2DConversion%2DKit%2DAmadeal%2DXJ12%2DMilling%2DMachine%2DAMA_SP_XJ12_CONVKIT%2Ehtml#SID=32
but the same issue with the spindle size would, I assume, apply. An email to them might clarify matters.
John
P.S. I’m not sure that Little Machine Shop will send to the UK, if that’s where you are, since the British Empire expelled Europe. You might get stung for import tax and VAT as well.