Can I use a 12V 70A source to convert/boost to 24V 5A supply?
Yes. The current through the converter is determined by the load. I doubt the monitor is pulling all 5 amps from the original supply anyway. Just throw an ammeter on the ground of the pc side and measure it to be in the safe side, but it should work without modification. With 100% efficiency the input would be, at max draw, 12v 10A, for 24v 5A. In free air inside the pc case, with fans you should be alright assuming 4A load draw