would a 3.25H-19 be the same size as a 100/90-19?

 

I think this tire guide will help you find what you need.

From what I've seen there isn't too much extra room in these swing arms. Measure the clearance with the current tire and that will give you some idea of what to try. It's often just an experiment.

 

There are resto Dunlops out there. I bought one a few years ago, can't remember if it was these guys or not. It matched my original '69 tire, except it did not say "Made in Japan". These guys show one:

Dunlop Motorcycle - Vintage K70 & K81

 

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I have a pair of Conti Blitz on my SR. Good handling tire as near as I can tell.