Dont you even try it. It will easily damage the spur gear and you'll have extremely poor speed if as you said the hole is eccentric. TAKE IT TO A LATHE SHOP AND RELIEVE YOUR MIND.
The name is Alawi. You can call me Al.
Nice to meet you!
I have a 6mm to 5mm reducer, but haven't found a 5mm to 4mm reducer yet. I am going to get a few 4mm bore pinions made the next time I get some done (very soon).