If you are keeping the stock rims, use as close to the stock sizes you can get. Metzeler and others still make the inch sized tires that the bike was set up to use. 3.25x19 Front and 4.00x18 rear, these have the proper aspect ratio. Probably need to have your tire shop order them, though. And, they will try their best to sell you whatever stock they have on hand.
Metric sizes are an approximate cross, and work ok when new, but have uneven wear issues during their useful life. However, they are unlikely to give the superb neutral handling these bikes were born having.
