If you look at chapter 25 of the Factory Service Manual (81 addendum) it shows a 17 inch rear for your '81 K model bike 4.50H17. So your wheel is correct to the original equipment.
According to various charts I've seen on the net, a 120/90 is the metric replacement for the old 4.50 "inch" tire designation.
Here is one example
TireChart.html but you can get several others on Google.
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Don't wait for opportunity to knock... kick the door down and drag the old harlot in!