forget modifying. either purchase something with the fat wheel already fitted, or start making a jig to build your own frame, using the car or motorbike tyre, as linked to above somewhere...
theyre completely impractical btw...
in other words, you look like a complete retard when you have to push one home
i cant stand these choppers people make that are completely unrideable... why waste all that time? pedals need ground clearance, and not only does it look stupid, but having your hands a foot over your head with your butt about level with your feet gets pretty uncomfortable after ten seconds or so, especially when you have to lean forwards to reach said handlebars... unless you walk around on all fours too?
stick to a normal bike so you can at least stand on the pedals when encountering a large unavoidable pot-hole...