Let me try to answer your questions.
1. What paint codes should be used for this tractor?
For the paint codes, please refer to the Paint, Decals & Other Finish ?
on the Manual Server. The codes are all there including cross-references to other paint manufacturers.
3. If I change the serial number plate out durring my restoration but keep the same serial number and old serial plate will it lower the value of my tractor?
I would think not. Many have done it. I would keep the original S/N Plate in a safe place. One that is safe enough, but not so safe that you forget where that safe place is. DON'T ASK
I also own a 48 cub and it has about an inch of varnish from old gas in the bottom of the tank. What is the best and cheapest way to clean this out? I was thinking of using "berrymans Chem Dip" but that may be expensive.
Simplest answer would be to take it to a Rad and Tank Shop. They should be able to clean and seal with Red Coat for around $75.00US. I had it done to Ellie-Mae my 48 and am so happy with it that Granny my 47 is going to get the very same treatment
I think it will also be cheaper than doing it yourself, and a heck of a lot safer.
Hope this helps