You could machine the outside of the barrel away if you are careful, the outside of the cast iron is a cast finish with lumps and bump on it. The cost to do it wouldn't be cheap and it is really hard to get a good finish that is round on the outside with the port holes already there. Cylindrical grinding is about the only way to get a good finish on the outside of a 2 stroke sleeve with holes in it.
If your handy with drills, files and die grinders you can buy a flanged sleeve and cut your own ports. I've used old GS1000 suzuki sleeves to do big bore DT style barrels but to cut the holes takes a couple of hours.