I had this problem with a Marin years ago, in my ignorance and impatience, I used a bit of emery cloth to sand down the 27.0 crown so I could fit a 26.4 crown race. It worked for years without problems after that, so I must have done an OK job. I wouldn't do it now though, I'd either get the forks machined properly, or buy the right headset.
I also tried a threaded rod to install headset cups, even in frames where the cups and frames were all correct it never really worked properly and I reverted to drifting the cups in using a piece of wood and a hammer. Later I bought a proper headset press in a sale; like this one:
The stepped shoulders on the press hold the headset cups square, so they go in straight and it works really well.
In any case, I doubt the 30.2 cups will have damaged the frame, so removing the cups and fitting a JIS 1" headset will probably work fine and I wouldn't bother with the threaded rod, like me you'll probably find the hammer and piece of wood technique works better. Also, if your Marin frame and forks are like mine, you'll need a very low stack headset; the steerer on my forks is only 30mm longer than the headtube, so the top lock nut doesn't engage on the steerer threads with headsets with a stack height much over that.