The cheapest (and therefore probably the worst) way to convert it would be to simply put a pulley on each axis and use that to connect stepper motors via timing belts, without changing the original leadscrews. There will be a lot of backlash and the maximum feedrate will be fairly slow, however it would still be use-able and enable you to CNC mill the parts for a better conversion using ballscrews. I put stepper motors on my mill this way, but never got round to changing to ballscrews since the router does most things the mill can do:

CNC Mill, Motor / Driver testing - YouTube