I got the aluminum sleeve installed by Millenium tech, the place that does all the nickasil liners. This was a 89 cylinder and they just happened to have the sleeve in stock. I don't know if they make these anymore or not. But honestly, the 284 cc (69 mm bore, 76 mm stroke) that I had would run neck to neck with this 72 mm bore motor and the beauty of the 284 cc is you can get a hot rods 4 mil stroker crank, a 69 mm wiesco piston, and have your stock jug ported all for a pretty fair price. It's easier to buy a 310 or a 330 and slap it on but I think it's cool to run the stock stuff and it's pretty amazing what kind of HP you can make out of the stock stuff and keep some good reliability.
If you are still interested in the big bore, I know that NW sleeve in Gresham, Or will install a 72 mm bore steel sleeve for about 150.00, then you will have it ported after this. I bought a hot rods 4 mil crank and had the YZ rod put on it by Hybrid Engineering in Camas, Wa. Total cost of the crank was $480.00 after purchase price and rod install. You can also have Crankworks do the same crank. You can run the KTM rod but I just like the YZ 490 rod/stroke ratio better. With a 4 mil crank it puts it back to 1.80, the same as a stock stroke long rod 250r. I also got the port timings I wanted with the KTM piston without having to deck the bottom of the cylinder to make the cylinder shorter due to the pin height of the KTM 300 piston.