Tractor Time With Tim has a good episode of this.
I have the Frontier 1149 for my 1025R.
It made mincemeat of my clay soil; it's like walking on Cool Whip (at least where the plants are).
The big measurements of a tiller are shaft speed (540 RPM seems to be standard) but the gearbox can drive the tine shaft at different speeds. Also, some tillers have 6 tines. I think my Frontier has 4 tines, and that's kinda standard. A "reverse" tiller runs backwards, i.e. it will pull your tractor backwards as opposed to pushing it. Also, some are gear driven and some are chain driven. Slip clutch or shear bolts?
I think there are tillers just as good as my Frontier, for less money. Land Pride and King Kutter come to mind.
I'm guessing that will fit your X series, but ask around. You gonna have enough lift height? How about room to connect and disconnect the PTO? I don't know of anything smaller than a 48" tiller, but they sure get bigger!