You can get the proper rim off ebay for less than $100, then you will not have any worries running a 150. You just need to look for a 03-06 fatboy wheel. I bet if you ask around on here someone has one laying around.
The real issue is the strut bolts. Make sure you turn them around...Bolts coming out and use acorn nuts...This way they are not sticking inside the fender risking shredding your tire..
My buddy has an '02 Fat Boy and he is on his 2nd or 3rd 150 HD Dunlop on a factory 16-inch wheel. It took zero modifications and clears the factory fender bolts to boot.
well they came with a wheel option. cast wheel had an mt90/130mm
profile wheel had a 150.
you could do a 150 I think ny bigger than that
you would prob run into issues hitting the belt.