This is (or at least used to be...) a common procedure. Unless you're intending to use a $12 headset on your bike, you might as well have a shop mill the crown race seat for you. Milling the headtube is usually a little more expensive, but also doable. Modifying a frame and fork to fit an ISO headset is not a big deal.

If you do decide to use a JIS headset, keep in mind that most of the headsets that are sold as "JIS" really just have a JIS crown race, and the headset cups are still 30.2mm diameter, rather than 30.0.

Modifying a crown race to a larger size only makes sense if you happen to have a lathe, and some patience, and lack a crown race seat milling tool or the $15 it takes to have a shop do it for you.