You could try and braze it ! However I suspect that it will warp and may even crack further. Even so you will still have a machining job to get it all straight afterwards.
In the old days it would be heat soaked in a furnace and then welded up whilst still in there, then afterwards allowed to cool slowly. Even so with luck and a fair wind you might get away with minimal distortion and machining.
I would secure the cracked piece in place with a few allen head capscrews down through the top. Contamination aside, braze does not penetrate cracks to begin with. Since there is so little strain on the guideway at the back end of the slide, little needs to be done to secure it, welding or brazing will do more damage than good.
andy, bolting it as john suggests the top of the ways where the bolt head would be is not a bearing surface and the chips would not hang up there and cause gouging of the ways. one other thing is to put a hole at the end of the crack to stop it from continuing along, do not put the hole in the bearing surface of the ways bill
Is that a topslide or cross slide way? If it was a topslide I was going to advise you to look for a new one as they might be easy to find. In fact a lot of guys are replacing the topslide (compound for us in NA) with a solid block to make the machine more rigid. But a cross slide is a different matter. Welding cast iron is tough because with the high carbon content it is brittle and can't take any internal thermal gradient without cracking. So as the previous poster mentioned, you can't heat up one area and have the adjacent areas still cold - it will suck the heat from the weld and also the internal stresses will cause a crack for sure. So you have to heat the whole thing up in say a bbq, insulate it with blankets, weld it, then cool it down gradually. There are newer nickel based welding rods that can weld the material quite well. And you can braze it. And some guys weld it with directly with a torch, flux and filler metal from cast iron piston rings (Kent White for one - but he is not your average welder). Then they put the part in a wood stuff with some logs and let it burn down over 24 hrs or back in the bbq and let it all cool gradually. But for an enco Chinese lathe you have to wonder if it is worth it. Try to get a replacement part if you can.
TIG silicon bronze, ala this old tony, looks like it is helpful for low distortion build-ups. Maybe a combination of the screw and silicon bronze. I have had some luck with soft solder. Keep all contact areas large using plates/pins, and keep the flame off the interface. Brownell Hi Force 44 is good stuff and flows well with the correct flux. One may say that it is only good for about 10000psi or so, but that goes a long way with a plate and/or pins.