Some of the most popular options are either the Bridgestone Spitfire S11 tires or the Battlax BT45 tires. It is hard to find tires in the original 3.25 x18 and 4.00x18 english sizes, but there are some out there. I've found that the Battlax 110/90 x18 works well on the back and very close to the original tire configuration. If you go with a 120/90 series on the rear, it might fit or might rub on the swingarm, but in any case, it will be more of a pain to remove the rear wheel for maintenance (must partially deflate tire for clearance under fender). This doesn't happen with the 110/90 series. On the front, you can go with the 90/90x18 (pretty close to the original profile or just under) or with teh 100/90x18 series (a tiny bit oversize, but fits fine and I have no complaints at all with it's handling, etc.) The BT45 tires are a bit stickier and I really like how they handle in turns. The S11 tires are good too, possibly slightly longer mileage with them but the bike is light enough that there likely isn't much difference.
Or you can go with the usual Metzlers, they have alot of good things about them, but I don't like how the front behave on scarified pavement, metal corrugated bridges, or tar snakes..