Hi - oddly enough I've been working on exactly the same thing this month.  Re-gauging the wheels was the easiest part... it's just 3/32  (0.09375) diameter brass rod . I cut lengths to 0.80 inch, rounded the ends with a file and replaced the old axles. I'm a little concerned the gears are going to slip with the removal and replacement - we'll see.
I've got access to a 3D printer (ABS) and I've been experimenting with inserts to widen the frame. Because the frames are Delrin, gluing really wasn't an option so mine snap over the whole frame. My third attempt is pretty promising, but it's binding on the couplings, there is a lot of wheel slipping and the contacts are shorting out on the frame - still, so far so good.

Pictures here
http://lynxmountain.blogspot.ca/2015/05/3d-printer-parts-for-hon3-shay-regauge.html