Just to help in your search, Phil Little Racing makes a kit to run a 19" mag as a rear. It's $300.
Member: BobHopper had one for sale a few months back if I remember correctly.
I have a 110 on my 19" mag for the front as well. I think the rim is 2.25" width, and I think a 120 would be pushin' it.
As far as gearing, that really depends on total tire height, just divide new tire height by old tire height, and that gives you your change in gearing.