I installed one of those Chinese made magnetic DRO units on a Harbor Freight 12 X 36, which is just like the Smithy. Installation was pretty easy- drill and tap some holes and fabricate shields etc. If I remember, I had about 8-10 hours in the whole job. However, the magnetic scales were a real pain- they are very sensitive to any liquids and they use shims to keep the distance between the scale and magnetic pickup. If you have any mis-alignment the scales can skip a beat and if you don't notice it, your measurements will be off. I fiddled with it for about a year before finally going with the Korean made Jenix unit with optical glass scales. They are spot on and very reliable. Also less expensive than the magnetics. Here's a link to their E-Bay site.
DRO JENIX USA 2 AXIS LATHES UP TO 14 X 40 599.00 | eBay