1 1/8" forks in a 1" headttube, have a look at my IS41 adaptor and let me know what you think!

Don't crack the cup though! It might be more of a risk if the headtube is slightly oval. I can't think of a way of making it more cylindrical without turning it on a lathe, but it might be worth half an hour with your best calipers and some very fine abrasive paper once you've taken the paint off.